Ayumi Hamasaki (thousand arms video game and, boys & girls video clip and tons of nice songs)
Utada Hikaru (Kingdom Hearts and tons of nice songs)
Maaya Sakamoto (in Escaflowne, too many anime themes, played Aeris in FF:AC and very lovely songs)
BoA (on my friends CDs, this group sings in so many different languages XD)
Do As Infinity (Inuyasha and some good neat songs)
The pillows (those who did some tunes for FLCL)
Black Mages (video game themes from FF series with Nobuo Uematsu)
Not much others...