This page had not been updated for years, but now that a sizable amount of shôjo manga has been translated into English, it seems a good time to
Here is a short and by no means complete list of shôjo manga that are widely considered to be "classics" and/or are personal favorites of mine. They are listed according to artist name in Japanese "A-I-U-E-O" order in the following format:
SURNAME Given-name, Title, Publisher, Magazine-it-appeared-in, date(s)-of-original-publication, number-of-paperback-volumes. Brief description.
Thanks to Kitsune for providing most of the comments (Her's are those followed by a "[K]"), and to Tony, for letting me filch one of his comments.
In some cases there may be more than one edition, with different numbers of volumes. Due to practical constraints, information is given for only one edition. If a title is accompanied by an image, the volume information is for the edition pictured.
AOIKE
Yasuko, Eroika yori ai o komete ("From Eroica With Love"),
Akita, Princess, ??? 1978 to ??? 1988, 19 vols. The first shôjo
manga to develop a following among English readers, this classic spy thriller
pairs two beautiful men who seem to provoke a certain, um, stimulation in
the imaginations of many female (and maybe some male) readers, both in Japan
and abroad. 
AKISATO
Wakuni, Nemureru mori no binan ("Sleeping [Male] Beauty"), Shogakukan, Petit
Flower, March 1986 to July 1986, 1 vol. One of the most realistic depictions
of gay life I've seen in a shôjo manga, and one of the first to deal
with A.I.D.S. 
ARIYOSHI
Kyouko, Suwan ("Swan"), Shueisha, Weekly Margaret,
#47 1976 to ?? , ?? vols. A classic ballet manga and a girl growing up story.
[K] 
IGARASHI
Yumiko, Kyandii Kyandii ("Candy Candy"), Kodansha, Nakayoshi,
?? 1975 to ???, ?? vols. Candy-Candy tells the story of an orphan, Candies
White. Rather than focus- ing on mushy first-love (although it does have it
here and there), Candy- Candy focus on Candy's and her friends' developments
as they grow into adults. Reading Candy-Candy, one can almost believe the story
really happened somewhere once. [Tony] 
IKEDA
Riyoko, Berusaiyu no bara ("The Rose of Versailles"),
Shueisha, Weekly Margaret, Spring 1972 to Autumn 1973, 10 vols.
A sweeping, epic romance that follows the tragic career of Oscar, a crossdressing
woman raised to be a warrior and doomed to die defending Marie Antoinette and
the French crown. 
IKEDA
Riyoko, Orufeusu no mado ("The Window of Orpheus"),
Chuokoronsha [originally Shueisha], Weekly Margaret/ Monthly
Seventeen, January 1975 to August 1981, 4 vols. Another historical classic
from Ikeda...involving, um, a crossdressing woman caught up in a revolution.
But this time it's the Russian Revolution! Any similarity to Berusaiyu
no bara is strictly coincidental. ("Aizouban" edition). 
IKENO
Koi, Tokimeki tunaito (literally, "Palpitation Tonight"),
Shueisha, Ribbon, July 1982 to ??, ?? vols. Dad's a vampire and
Mom's a werewolf, but the daughter is just an ordinary highschool girl, or
so she believes. [K] ICHIJOH
Yukari, Dezainaa ("Designer"), Shueisha, Ribbon,
February to December 1974, 1 vol. Intense! Once you get past the goofy 'seventies
fashions, there's no escaping this tragic tangle of hatred and love. 
ICHIJOH
Yukari, Yuukan kurabu ("The Leisure Club"), Shueisha, Ribbon/ Ribbon
Original/Chorus, Spring 1981 to present, 16 vols. to date. OHSHIMA
Yumiko, Banana bureddo no puddingu ("Banana Bread Pudding"),
Shueisha, Weekly Seventeen, 1977, 1 vol. (From Asahi Sonorama)
Now that Ira's older sister is getting married, who will take her to the bathroom
after ten p.m. and sing "Twinkle, Twinkle Little Star" outside the
bathroom door in order to protect her from the beautiful, androgynous, child-eating
clown? Will marrying a closeted gay man help? Matt's personal favorite. 
OKANO
Reiko, Fanshii dansu ("Fancy Dance"), Shogakukan Petit
Flower, October 1984 to July 1990, 9 vols. If you're into 'eighties
New Wave and/or Buddhism, you might enjoy this quirky, funny tale of angst
and dharma. 
OZAKI
Minami, Zetsuai 1989 ("Desperate Love 1989")/Buronzu ("Bronze"),
Shueisha, Margaret, September 1989 to present, 11 vols. to date.
Why must I be a teenaged pop idol in love...with another boy? Love knows no
gender. It might even drive you to hack your own arm off. If you like your shônen-ai (or "slash")
intense, look no further. 
KAWAHARA
Izumi, Warau mikaeru ("The Smiling Michael"), Hakusensha, Flowers
and Dreams, #3 1987 to , 3 vols. The girls who just cannot find their
places in the most elegant and distinguished private girls' school quickly
learn that they are not alone—in fact, there are three of them. [K] 
KIHARA
Toshie, Yume no ishibumi ("The Stone Tablet of Dreams"), Shogakukan, Petit
Flower, May 1984 to present, 16 vols. to date. 
KIMURA
Minori, Poketto no naka no kisetsu ("The Season in My
Pocket"), Shogakukan (Shueisha/Mushi
Productions/amateur), Special Edition Girls' Comic/ Ribbon
Comic/Ribbon/COM/ Monthly Funny/amateur
publication, September 1969 to January 1976, 1 vol. 
KUBO
Kiriko, Shinikaru hisuterii awaa ("Cynical Hystery Hour"),
Hakusensha, LaLa, March 1982 to ???, ?? vols. 
KURAMOCHI
Fusako, Itsumo poketto ni Shopan ("She Always Has Chopin
in Her Pocket"), Special Edition Margaret, February 1980
to July 1981, 2 vols. First shôjo manga (well, maybe not) which features
classical music and the protagonist actually plays piano. [K] 
CLAMP, Toukyou
babiron ("Tokyo Babylon"), Shinshokan, WINGS/South,
August 1990 to winter 1993, 7 vols. A boy exorcist SUMERAGI Subaru's story...
continues to X (another manga by CLAMP). [K]
KOUGA
Yun, Aashian ("Earthian"), Shinshokan, WINGS/South,
September 1987 to Spring 1995, vols. Angels helping out humans, or evaluating
them. The plot is popular with many Japanese mangaka, from ISHIMORI Shohtaroh's "Nikyuu
tenshi"(dattakke?) to TEZUKA Osamu's "Ribon no kishi". This
particular one adds a hint of yaoi. [K]
GOTOH
Yukio & MAKINO Kazuko, Haitiin bugi ("The Late Teens'
Boogie"), Shogakukan, Petit
Seven, ??? to ???, ?? vols. A "growing pains" story about
two highschool kids. The name "Shoh" (as in 'hishou') is quite popular
among Japanese yan-mamas, but few of them know that it comes from this manga. [K]
SAKURA
Momoko, Chibimaruko chan (""), Shueisha, Ribbon,
August 1986 to ???, ?? vols. A biographical manga about SAKURA Momoko as a
third grader, one can read it and learn a bit about the life of a little girl
in Japan in early 70's. [K] 
SASAKI
Noriko, Doubutsu no o-isha-san ("Animal Doctor"),
Hakusensha, Flowers and Dreams, January 1988 to ??? 1994, 11 vols.
Hilarious, deadpan humor, set in Hokkaido University's veterinary school, and
based largely on background research the artist conducted there. 
SHITOH
Kyouko, Henkyou keibi ("Guarding the Frontier"), Shogakukan, Petit
Flower, March 1988 to May 1992, 6 vols. A chronicle of an imaginary
country by the frontier, where dwell ordinary people, young and old. [K] 
SHIMIZU
Reiko, Mirukii uei ("The Milky Way"), Hakusensha, LaLa,
April to May 1986, 1 vol. First of Jack & Elena series, where Jack meets
Elena. The two are robots. They can live forever. Do they want to? Maybe yes,
since they found each other. [K] 
SHOHJI
Yohko, Seito shokun! ("Attention Students!"), Kodansha, Shôjo
Friend, 1977 to ??, 24 vols. 
TAKAGUCHI
Satosumi, Hana no asuka gumi! ("The Glorious Asuka Gang!"),
Kadokawa, ASUKA, August 1985 to ???, ?? vols. Asuka used to be
an ordinary girl in junior highschool until one day, to the surprise of the
loving mother & father, she learns to fight back. [K] 
TAKEMIYA
Keiko, Kaze to ki no uta ("The Song of the Wind
and the Trees"), Shogakukan, Weekly Shôjo Comic,
January 1976 to ??, ?? vols. The shônen-ai classic that was
the first commercial shôjo manga to show two boys in bed...and they
weren't sleeping. Heart-rending and beautiful. TSUCHIDA
Yoshiko, Tsuru hime jaa! ("It's Princess Tsuru!"),
Shueisha, Weekly Margaret, 1973 to ??, ?? vols. 
TSUMUGI
Taku, Hotto roodo ("Hot Road"), Shueisha, Special
Edition Margaret, Jan. 1986 to May 1987, 4 vols. Tsumugi captures the
complex and subtle moods of a group of marginalized teenagers with extraordinary
sensitivity. The influence of this master of minimalism on later artists can't
be overstated. 
NASU
Yukie, Koko wa Guriin-uddo ("Here is Greenwood"),
Hakusensha, Flowers and Dreams, ??? to ???, 11 vols. Ryokuto Gakuen
is the private boys' highschool, and Greenwood is the school dorm. If you have
so many boys you bet you can get more than a few weird ones. [K] 
NARITA
Minako, Eirian sutoriito ("Alien Street"), Hakusensha, LaLa,
December 1980 to ???, 8 vols. 
HAGIO
Moto, Tooma no shinzou ("The Heart of Thomas"), Shogakukan, Girls'
Comic, May to November 1974, 2 vols. The manga that changed my life.
In a German "gymnasium," a boy struggles with the death of a boy
who loved him by coming to terms with a new transfer student...who looks just
like the dead boy. 
HIJIRI
Chiaki, Iki ni yarou ze iki ni yo ("Let's Do It With
Style"), Shueisha, Special Edition Margaret, July 1987 to
???, ?? vols. In a typical shôjo manga, a heroine is just another girl-on-the-block,
and she falls in love with a boy of her dreams. Here our heroine Tomako is
the school queen, and she falls for...now, who was that again? [K] 
HIWATARI
Saki, Boku no chikyuu o mamotte ("Protect My Earth" or "Please
Save My Earth"), Hakusensha, Flowers and Dreams, Jan. 1988
to ?? 1994, 21 vols. To be honest, I thought it was going to be silly. I was
wrong. Hiwatari takes the most implausible story and makes it utterly convincing,
drawing you into a complex web of friendship, resentment, madness, jealousy.
I cried a bucket of tears before it was over. 
HOSOKAWA
Chieko, Ouke no monshou ("The Crest of Royal Family"),
Akita, Princess, October 1976 to present, ?? vols. 
MIUCHI
Suzue, Garasu no kamen ("The Mask of Glass"), Hakusensha, Hana
to yume, January 1976 to present, ?? vols. MIZUNO
Hideko, Faiyaa ("Fire"), Shueisha, Weekly
Seventeen, January 1969 to July 1971, 3 vols. Although the theme of "freedom" seems
corny today, after nearly thirty years Mizuno's skill and artistry still shine
through in the first shôjo manga to deal with sex, drugs and rock 'n'
roll (not necessarily in that order). 
MICHIHARA
Katsumi & MAKI Yuu, Teiou no niwa ("Garden of the
Emperor"), Shinshokan, WINGS, March 1986 to May 1987, 1 vol.
The first tankoubon of the seires "Joker", about an artificial-human
cop Joker and a human cop who fall in love with each other. Sounds like romance?
It's more comical than romantic, especially when Joker can transform into just
about anybody, male or female. [K] 
YAMAGISHI
Ryohko, Hi izuru tokoro no tenshi ("Heaven's Son in the
Land Where the Sun Rises"), Hakusensha, LaLa, April 1980
to ??, 8 vols. (from Kadokawa Publishing) Yamagishi turns conventional historical
wisdom on its head in this gripping, beautiful, and frightening drama set in
the dawn of Japan's history as a nation-state. It was a classic before the
ink of the last page was dry. 
YAMADA
Mineko, Harumagedon densetsu ("The Legend of Harmageddon"),
???, Duo, September 1981 to March 1985, 6 vols. I probably shouldn't
recommend people this manga, simply because once you get hooked to this "Harmageddon
series" you'll have to read almost all of her works. Anyways, this particular
story is about Nagato and how he becomes the king of Soma. [K] 
YAMATO
Waki, Haikarasan ga tooru ("There Goes the Modern Girl"),
Kodansha, Shôjo Friend, 1975 to ??, 8 vols. 
YOSHIDA
Akimi, Kariforunia monogatari ("A California Story"), Shogakukan, Special
Edition Girls' Comic, February 1978 to ???, 4 vols. **comments space
reserved for Akemi-san^^** 
WAKATSUKI
Megumi, Soo watto? ("So What?"), Hakusensha, LaLa,
August 1986 to July 1989 , 6 vols. A girl receives a telegramme—"Your
grandfather is gravely ill"; hops on the train and rushes to his home,
opens the door and — the story begins. Ari and her ghost grandpa's home
welcomes anyone, young and not-so-young, human and aliens alike. [K] All images copyright their respective artists and/or publishers.
NEKOBE
Neko, Kingyo chuuihou! ("Goldfish Warning!"),