Just to help out, I listened to each one of these people at least once and most a relatively unknown I guess. Found these while I was downloading random songs for fun:
Ayumi Hamasaki
HIGH and MIGHTY COLOR
Akiko Kawakami
Anna Tsuchiya
Crystal Kay
Eiko Shimamiya
Orane Range
Aya Hirano(Look for songs other than God Knows and Hare Hare Yukai. They're good but there are more.)
Rika Matsumoto(Yeah, she makes some Pokémon songs.)
Can/goo
UVERworld
Rie Fu
BoA
HOME MADE KAZOKU
KAORI(Yeah, she made more than I Won't Lose.)
Hitomi Takahashi
L'arc-En-Ciel
Aqua Timez
Hikaru Utada(My absolute favorite. I got 66 of her songs and she sings in English really good. Likely, because she was born in America. She made the Kingdom Hearts songs.)
MOVE(Their English isn't that great but their Japanese is.)
Shouko Nakagawa
Note: All names and in the English format( First Name, Last Name).