Cristiano Paris <email@example.com> has done some great work in updating the original mod_tproxy code to work with newer Apache sources, and also has overcome some obstacles with compiling as a dynamically loaded module. Additionally, he has put together a website at http://lupo.campus.uniroma2.it/progetti/mod_tproxy/ where he includes better instructions, and his updated code.
I highly recommend visiting his site for the latest and most stable code.
This module was written by me in 1997. I haven't touched it since then. I've been using it, however, since that time, and it has worked flawlessly, on a network with about 25 users in a web shop, who have healthy web browsing appetites. It has been running on a (now old) K6-200 machine, with a max uptime of about 500 days or so.
It requires linux' transparent proxy support. The BSD's may likely have a similar mechanism. If you port it to work with another OS, send me a patch, and I'll include it here.
This module is contributed to community under the Apache License, BSD licence or GPL, whichever you like.
If you find this useful, let me know. It's nice to hear about helping someone once in a while..
Steve Kann SteveK@SteveK.com