Wippien.pl

Wippien is an open source (however, including proprietary non-free components) P2P zero-configuration VPN client. It is capable of establishing direct links between computers behind NAT firewalls without requiring any router configuration at all (in most cases); in other words, it emulates a local area network over an established Internet connection. It is quite similar to Hamachi but uses XMPP to manage user accounts, thus enabling features like a contact list, avatars etc. Rather than Hamachi's "network" approach, Wippien requires users to directly connect to one another. (en.wikipedia.org)