Windows/Mac: Libox makes it easy to share music, movies and photos with your friends in a private P2P network.
After installing the Libox media browser and importing your media, you can share with friends — they must be using the Libox browser also — as easily as if you were sharing a link to a collection of pictures online. Your data isn't stored on the Libox servers, their servers just facilitate a connection between your client and that of your friend.
You can also use Libox to sync media between your own computers. Check out the video to see it in action:
A very alpha feature allows you to browse your collection through a web browser instead of the desktop client, but currently only works if you're not blocked by a firewall so it's limited to local network browsing.
Have another method of easily sharing files with friends in an private network? Let's hear about it in the comments.