Right now I am listening to the One Piece soundtrack. It was so very hard to find. All I could locate at first were the opening and closing songs but that's not what I wanted! I love the background music. I finally found it and LOVE it.
Namidaboshi - Black Cat ED01
Filthy Defiler Disgrace - Chaos;head OP
Haritsuke no Misa - Chaos;head IN03
Super Special - Chaos;head ED
Magg Mell - Clannad OP
Toki wo Kizamu Uta - Clannad ~afterstories~ OP
Again - FMA Brotherhood (although i haven't watched that anime)
Almost all K-On! Soundtracks..
Almost all Kawada Mami-san no Uta..
Almost all KOTOKO no Uta..
Bouken Deshou, Hare Yukai, God Knows, Lost my Music..