CD Baby does a good job of it. I've got my album digitally distributed through them, and they put me on Lindows, Napster, DiscLogic, Rhapsody, MusicMatch, BuyMusic, Apple iTunes, MusicNet, & AudioLunchbox so far, with many more to come.
Basically they take care of the paperwork and hassle and take a cut when you sell something. Just like any other kind of distributor. CD Baby's got a long, good track record of artist-friendliness, so I'd recommend them over any others.
