Seiyuus:
Ayako Kawasumi - Mahoro, Saber, Nodame, Aoi (Ai Yori Aoshi), Kokoro (DoA)
Horie Yui - Naru, Hitomi (DoA), Kotori (D.C.), Tamami (Mahoraba), Ayu (Kanon), Yume (D.C. II), Minori (Toradora)
Ami Koshimizu - Horo (Spice and Wolf), Himari, Tenma (School Rumble), Yuumi (Kimikiss)
Singers:
Lia, CHATA, Kotoko, Yangai Nagi, Shimotsuki Haruka, Hatsune Miku, Megurine Luka, GUMI
Other Artists:
Casiopea (Jazz Fusion), T-Square (also Jazz Fusion), Kaori Kobayashi, Inspirits (though thats basically Casiopea), kz/Livetune (Electropop), OSTER Project, DECO*27, SwingHolic
I'm sure there's more, but I can't name them off the top of my head right now...as you can tell, I like Vocaloid and Jazz xD