Tsukumo, Mutsumi

A classic yuri manga about a woman who leaves her husband for an old friend from school. There are two stories in this manga--one (Moonlight Flowers) is the main story, and the other (Midnight Flowers) is about the protagonist's friend's past.
Note: this manga contains rape.
(Source: Lililicious)

Also includes another unrelated story.

Moonlight Flowers

月下美人

Amamiya, Sae

Plica-chan is the bittersweet, realistic story of Plica, a young lesbian looking for love in modern Japan.

The manga, which is written in 4-panel strip style, originally ran in Anise magazine, and now runs in the LOUD News, the newsletter for the LOUD organization. It can also be found online. Plica's comments are often on the nature of sexuality, the relationships between men and women, and between women and women.

The whole strip starts on a low note, as Plica returns from an ex-lover's wedding. But not too long into it, Plica meets Mari, for whom she starts to have feelings pretty soon.

(Source: okazu.blogspot.com)

Plica-chan

プリカちゃん

Takashima, Rica

The adventures of college student Rica, as she attends university in Tokyo and visits the gay and lesbian district looking for friendship and love. Rica's story is sweet and funny, realistic and a little wacky with a refreshing lack of self-hatred and doubt, but with no lack of real emotions.

(Source: MU)

Rica tte Kanji!?

Tokyo Love: Rica'tte Kanji!?

リカって感じ!?

Shiina, Ayumi

Takasaki Tomoka is a young girl seeking love. She had finally found her prince and slowly began to think of a way to speak to him. Tomoka met up with her childhood classmate from piano lessons, Miyazawa Arata and was invited to a performance by his band. There, she met the prince she had been admiring for many days. After becoming drunk, Tomoka confessed her love to her prince but problems arise with Tomoka having to take her love down through a dangerous path. (Source: M-U)

Anata to Scandal

You & Scandal

(あなたとスキャンダル

Wako

WAKO's "Presents on Christmas Eve" (or, as Syk and I prefer to call it, "All I Want For Christmas is Surprise Lesbian Sex"), is about learning to overcome the pain of the past and live in the present. Or, as we prefer to describe it: "You raped me? How could you do that, that's so awful!" "But I'm a woman." "Oh, OK, then. Lesbian rape is great. Can I have your name now?" "No. Less talk, more sex!"

(Source: Lililicious)

Seiya no Okurimono

聖夜の贈り物

Loneliness... They meet on a park bench one night. They were just classmates, barely knew each other. Secrets... Something make them talk to each other, reveal truths about themselves. Warmth... In the chill night kisses and caresses are more than just lust, as they explore and discover. Love...? 

(Source: Baka-Updates)

Dame Ningen Jan

Matsufuji, Junko

In Eve no Onna-tachi, by Matsufuji Junko, Aiko is unhappy in her marriage—her husband only married her to get ahead at work, and he's hardly ever home. Luckily, she has Kyouko, a woman she met online one lonely night. But when Aiko's husband suspects she's having an affair, their vision of a happy Christmas Eve together is looking less and less likely.

(Source: Lililicious)

Eve no Onna-tachi

イブの女たち

Sugio, Hisako

"Friends" tells of a promiscuous woman who realizes she's fallen in love with her friend.

(Source: Lililicious)

Friends

Sakura, Shio

One of the Yurimm Fairy Tales, it is about a womanising wolf-girl and seven goat sisters. It is more an illustrated story than a manga.

The Wolf-Girl and the Seven Kids

Oda, Mei

In "Anniversary," a couple makes love for the first time. (Source: Lililicious)

Anniversary

Mariko

Yayoi gets dumped, but then meets someone who understands her in a way her ex never did.

(Source: Lillicious)

Ningyou-tachi no Heya

人形たちの部屋

Minami, Q-ta

A collection of oneshots including "I Say a Little Prayer":

Kiyoko is in a bar one day, drowning her sorrows, when she runs into a girl who used to have a crush on her in school, Ayumi. They end up hooking up. Kiyoko knows she's gay, but Ayumi has never been with another woman before. Will they manage to work things out? (Source: Lilicious)

12 Manga Like Girl's Only: Manga Carmilla