I just want to buy the occasional track and typically not albums and would prefer MP3 for maximum compatibility and simplicity. The only ones I'm aware of are:
iTunes (AAC only!)
What other sites are there that sell single tracks and have very large libraries? I listen to mainly non mainstream music such as Jazz & World Music if that makes any difference.
In the past I had a Napster subscription and found that very useful as a way to listen to a very large range of music without buying it all.