Whoo, way to bring this thread back alive!

Bleach: Younha - Houki Boshi
Blood+: Hitomi Takahashi - Aozora no Namida
Blood+: UVERworld - Colors of the Heart
Code Geass: Flow - Colors
Eureka Seven: Flow - Days
Full Metal Panic: Mikuni Shimokawa - Tomorrow
Full Metal Panic: Fumoffu?: Mikuni Shimokawa - Sore Ga, Ai Desho
Full Metal Panic: The Second Raid: Mikuni Shimokawa - Minami Kaze
Gundam SEED: Nami Tamaki - Believe
Gundam SEED: Nami Tamaki - Realize
Gundam SEED Destiny: High and Mighty Color - Pride
Gundam SEED Destiny: Hitomi Takahashi - Bokutachi no Yukue
Jyu oh Sei: Younha - Te wo Tsunaide
Kiba: Younha - Hakanaku Tsuyoku
Nodame Cantabile: SUEMITSU & THE SUEMITH - Allegro Cantabile
Rozen Maiden: Ali Project - Kinjirareta Asobi
The Melancholy of Haruhi Suzumiya: Aya Hirano - God Knows...
The Melancholy of Haruhi Suzumiya: Aya Hirano - Lost My Music

However, it doesn't necessarily mean that I've seen the series or liked it.