I have been using Musicmatch Jukebox, which is very similar to Napster, for several years now. About a year ago, Musicmatch added an online store.
The big difference is that Musicmatch was recently purchased by Yahoo! It's no secret that Yahoo! wants to be the next media mogul on the street. The acquisition of a tool that immediately places it on the same level as iTunes is a no-brainer. All that remains to be seen is how Yahoo! will leverage this relationship.