This is a short one, mostly so I don't forget where to find this tool later.
I needed to monitor some SOAP traffic and do some debugging... just wanted something small, won't take over my work laptop (Win XP) and just show me the HTTP messages raw. Enter TcpTrace, a small footprint http proxy that comes in handy when you need to do some troubleshooting at that level. Great for junior folks to actually learn something about HTTP protocol and for sticking on your thumb drive and taking with you.
This tool is for windows only, there are plenty for *nix out there... what's your favorite?