Instead of downloading songs onto your phone over EV-DO or pre-loading them via your computer, Mercora M lets you access your entire music library streamed over your broadband connection. The upside is that you have all your music all the time.
The software's only available for Windows Mobile 5 devices now, the Mercora M service will also (in the future) allow you to connect to your friends' music and listen to digital radio as well. The only problem we see is when spotty reception kills your stream, which pretty much renders the music unlistenable. It's going to cost you $4.99 a month or $49.99 for a one year plan.
Product Page [Mercora]