What it does
With ToxTun, you can create a virtual network with your friends with just one click. Ones the connection is established, your computers behave as if they were in the same LAN. So you may share files, play LAN games, use shared printers etc.
What it can't do yet
At the moment, only one to one connections are supported. Support for group connections is planned.
A minimal Tox client (fToxTun) with support for ToxTun is available for download (build with libToxTun 0.0.2):
To use it on Windows, you need to install the tap driver of the OpenVPN project. You can find it here (look for Tap-windows).
If you want a full featured Tox client, you can find sources for qTox with added ToxTun support here, but you will have to build it yourself for now.
How it works
ToxTun uses Tox's API to communicate with your friends. So all traffic is encrypted and authenticated by Tox.
When you try to connect to a friend, your friend is asked whether or not he wants to accept the connection. After he accepted, ToxTun selects an 192.168.0.0/24 subnet that isn't in use at both PCs (for example 192.168.17.0/24) and sets up the interfaces accordingly. (On some versions of Windows it will fail to set the IPv4 for you if it is not running with administrator privileges. If this is the case, you can still configure the IPv4 manually.) IPv6 is also supported and should work out of the box with the auto generated link-local addresses.