Monopod is a simple Podcast client. It's for people who want to select a few channels, come back later, and find the the Podcast MP3s turn up on the hard disk.
Stroom is a music library that allows you to share your music over the internet. It comes with a Winamp plugin, a database management/import tool, and in the future also a web interface. It supports mp3, ogg, and various other formats.
A DAAP (Digital Audio Access Protocol) client in .Net which can connect to any DAAP server sharing music on a network (such as itunes). Intended for PocketPCs to stream music, but should run on anything that runs .Net
Last Dot FM is an alternative open source player for last.fm web service. It supports all features of official client and also some additional features. Written in .NET 2.0.