I'd totally recommend:
Code Geass OP: COLORS (by Flow)
Melancholy of Haruhi Suzumiya: God Knows... (by Hirano Aya)
Hatenkou Yuugi OP: Heartbreaking Romance (by Kanako Itou)
Vampire Knight OP: Futatsu no Kodou to Akai Tsumi (by ON/OFF)
Shugo Chara ED: Honto no Jibun, Renai Rider, Kiss! Kiss! Kiss! (by Buono!)
Jigoku Shoujo OP: Sakasama no Chou (by SNoW)
D-Gray Man OP: Doubt & Trust (by access/axs)
xxxHolic ED: Changing (by Bon Bon Blanc)
Chrno Crusade ED: Sayonara Solitaire/Solitia (by Saeko Chiba)
Lucky Star ED karaoke themes: Cha-la-Head-cha-la(by Hirano Aya as Konata Izumi), Valentine Kiss (by Fukuhara Kaori as Tsukasa accompanied by Konata)
Round Table featuring Nino have sung for various anime OP & ED themes so you might wanna check their discography since they've got great music. Hirano Aya sings for various anime OP/ED too

Some seiyuus sing the osts for their anime shows (eg. Miyano Mamoru, Fukuyama Jun, Hirano Aya, Minori Chihara, etc)