bsdproxy is a generic, event-driven proxy designed specifically for the
BSD platform.  It uses the kqueue()/kevent() system calls to determine
when to relay data from one side of the connection to the other.  It also
uses GLib (http://www.gtk.org) data structures and memory management
functions to optimize steady-state performance (minimize unnecessary
memory allocation/deallocation).

bsdproxy has been used to proxy HTTP, HTTPS, telnet, and mysql without
any problems.  It should be able to serve as a transparent proxy for
anything over a TCP/IP connection.

WWW: http://daveb.net/bsdproxy/