Proxy Madness - What's YOUR Flavor?

OK, so proxies of all sorts--HTTP, SOCKS, or application-specific--are pretty much a fact of life in today's workplace.  With the advent of cloud computing, however, those heretofore lonely middlemen are once again major players in the troubleshooting/debugging world.  I'm currently working on several issues around web proxies and/or proxy authentication, so I'm curious...what proxies are you (or your customers) using?

I'll admit to an ulterior motive.  I'm in the process of building a proxy server for testing purposes, and I'd like to include as many proxies as I can.  So far, I'm ready to install Squid, Apache, and (after digging through my source code archive) the reference implementation of SOCKS5; with authenticating and anonymous instances of each.  I'll have NTLM authentication available, as well.  Can you guys add any others to the list?