My personal favourite anime OST :

FMA :
- Motherland by Crystal Kay
- Melissa by Porno Graffiti
- Tobira no Mukou e by Yellow Generation
- Kesenai Tsumi by Nana Kitade
- Heck... almost all of FMA's music and songs ROCKS!

Ayashi no Ceres
- Scarlet by Iwao Junko

Fruits Basket
- For Fruits Basket
- the instrumental pieces are very soothing. Highly recommended.

Onegai Sensei
- Sora no Mori de by Kotoko

Otogizoushi
- Hoshi ni Negai wo by Chieko Kawabe
- This song is probably the only thing I don't dislike about the anime.

Rozen Maiden
- Hikari no Rasenritsu by uh.. forgot the name.
- The music is pretty funny.

Wolf's Rain
- Gravity by Maaya Sakamoto

Gundam Seed
- Find the Way by Mika Nakashima.