Word of warning: Sending invites really sends email. Oops.
I stumbled upon Qnext today. It’s a java-based cross platform p2p tool that allows you to share files, music, pictures as well as email and IM (including irc) right from the application. It’s closed software but obviously comes with the jar files. It allows for group sharing and several other interesting things. Surprisingly, no jabber support. Firefox support is also lacking.