Samurai X
Afro Samurai
Gintama is a comedy
Samurai Champloo is one of a kind. I have yet to come across another anime that was so chill and blended in such music. Definitely a modern day classic
Cowboy Bebop is a different genre but kinda falls under chill with a nice soundtrack
Kids on the Slope - written by Watanabe, animated by MAPPA/Tezuka. It's about jazz music, friendship, and a little bit of romance. It's a very, very solid anime.
Baccano! - It's about immortal mobsters during the Prohibition in the USA (watch it dubbed, obviously). The OST is amazing and so is the way that the story is told.
Those are the only two shows with an amazing OST that I can think of right now. I'm sure there are others I could think of that you'd like, though.