CLAMP

CLAMP is a group of four women: Ageha Ohkawa (leader and script writer), Mokona, Tsubaki Nekoi (both art) and Satsuki Igarashi (art assistant). They began in 1989 as a group of 12, but eventually ended in the four there are now. In 2004, they changed their original names in the names they have now. Ageha's first name used to be Nanase, Mokona used to be Mokona Apapa, Tsubaki was Mick and Satsuki's name was written in different characters. This pas part of their 15th anniversary as CLAMP.
Current known works of CLAMP are Tsubasa: RESERVoir CHRoNiCLE, xxxHOLiC and Kobato., and they are probably most known for their works CardCaptor Sakura and Chobits.

Arina Tanemura

Arina Tanemura debuted at age 18 in 1996, in the magazine Ribon, to which she's stayed loyal for all the years that followed. She's best known for the manga Kamikaze Kaitou Jeanne and Full Moon o Sagashite, which were both translated to English and also both made into popular anime. Other works of her contain I*O*N and Time Stranger Kyoko, which were not published in English (but were in Germany). Her current manga is Shinshi Doumei Cross, which is also licensed to be translated in English.

Asuca Hayashi

Asuca Hayashi is my #1 favourite J-pop artist. She debuted at age 13, with the song ake-kaze, and got known better with Chiisaki Mono/Make a Wish. This was the ending theme of the sixth Pok�mon movie, and even the English version still contained half of her Japanese singing! Her voice is very mature, especially for a Japanese girl of her age.
Asuca has now released three albums (Saku, Hatsukoi and Chou), a minialbum (Tsunaide) and around twelve singles. Her latest one is Kokoro no Mama ni, which released at March 21th.
I'm a huge fan of Asuca, but I really have the feeling she's underrated. So please, if you can, sponsor her!

KOKIA

KOKIA's real name is Akiko Yoshida, and her artistic name is her first name read backwards, but also the name of a flower that blooms at her birthday July 22. She debuted with the single Aishiteiru kara, in 1998 at age 22. Arigatou was the first song which she wrote and composed all by herself. Many of her songs, including Arigatou, were used as theme songs for (animated) movies and series. Her first album was Songbird.
In 2006, KOKIA hit Europe and sung in France. That same country released her latest album Ai ga Kikoeru, but it is unknown when it'll be released in Japan. As of now, KOKIA's released 6 albums and 14 singles, and she appeared on quite some soundtrack CDs.
I got to know KOKIA by accident, when Last.FM recommended her as a similar artist of one of the other artists I listen to. And I surely don't regret looking up her songs!

Maaya Sakamoto

Maaya Sakamoto debuted at age 16 as both a seiyuu (voice actor) and a singer, when she voiced in the anime Vision of Escaflowne and sang some songs of it. Ever since, she continued as seiyuu and singer, and has published five albums and quite some singles. Maaya is pretty popular, also outside of Japan. Probably because her English is very good (like in the all-English songs Gravity and Afternoon Repose).
Maaya's voice is very clear and easy to listen to, in my opinion. I'd certainly recommend her!