VTun provides the method for creating Virtual Tunnels over TCP/IP
networks and allows to shape, compress, encrypt traffic in that
tunnels.

Supported type of tunnels are: PPP, IP, Ethernet and most of other
serial protocols and programs.  VTun is easily and highly configurable,
it can be used for various network task like VPN, Mobile IP, Shaped
Internet access, IP address saving, etc.  It is completely user
space implementation and does not require modification to any kernel
parts.

WWW: http://vtun.sourceforge.net