I use Apples i-Tunes alot, great selection and a reasonable price.
The downloaded files are .m4a
These are encrypted, and I can't find a direct conversion tool to make them mp3's. But the i-tunes software will burn a regular audio CD from a playlist - so I do that then rip the CD to mp3. Thats about the only easy way I have found to get downloads from my powerbook to my PC based jukebox. (70Gb of mp3's, 8,000 tracks

)
I use Music Match Jukebox for the front end and database.
Oh, and at the moment you need to have a Mac to access Apples i-tunes.
- Krynos