Grouper

Grouper is a P2P app with a difference. You can create private groups and share files with specific users. For example, you can configure the software such that your friends (and only your friends) can access your music collection. This is different from normal P2P apps, where any user using the same P2P app as you, can download any shared file from you. All data transfer (including chat) is encrypted.

Disadvantages are :

1) The program is still in beta, and it might have a few bugs.
2) The program requires the .NET framework, and is a little slow compared to a native Win32 program.
3) Downloading of MP3 and WMA files is not allowed, to curb piracy and stay in good books with rhe RIAA. Renaming the MP3 or WMA file (or zipping the file) acts a workaround to this, though.

But a useful, and usable program overall.

Check it out here : http://grouper.com

And if anyone wants to share files with me using this software, leave a comment here, and I'll get back to you :)