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BadVPN is an open-source peer-to-peer VPN system. It provides a Layer 2 (Ethernet) network between the peers (VPN nodes). The peers connect to a central server which acts as a chat server for them to establish direct connections between each other (data connections). These connections are used for transferring network data (Ethernet frames), and can be secured with a multitude of mechanisms.

Features:

UDP and TCP transport
Converges very quickly after a new peer joins
IGMP snooping to deliver multicasts efficiently (e.g. for IPTV)
Double SSL: if SSL is enabled, not only do peers connect to the server with SSL, but they use an additional layer of SSL when exchanging messages through the server
Features related to the NAT problem:
Can work with multiple layers of NAT (needs configuration)
Local peers inside a NAT can communicate directly
Relaying as a fallback (needs configuration)