taprouter

taprouter is a tool that makes it possible to use SLIRP connectivity over a virtual L2 TAP interface.
It allows you to get internet connectivity from within any TAP interface. The main use case is to provide networking to virtual machines or alike (I specifically wrote taprouter to make networking in DOSemu much easier, but it might have other interesting usage, too).
taprouter comes with its library companion - librouter, that provides the api for usermode networking (and could as well be reused in other opensource projects).