Well, anonymizer.com is not *really* anonymous, though they make that claim. When you use them, your web browsing is as anonymous as one-layer 128-bit SSL for anyone with a packet sniffer on the network. 128-bit SSL has been broken. And the company keeps track of all your web connections, any identification of you, payment information, all IP addresses, etc., and they will surrender everything upon any court's order. Anonymizer.com itself can also be hacked, though it's probably very difficult.

If you want true anonymous web browsing, you need to use chained proxy servers sending multiple-layered encrypted traffic to remailer servers. The U.S. Navy Office of Naval Research developed a very secure system for doing this and they even made an early version available to public access for a while. It's all open technology right now but they might later plan to patent their system to keep it out of commercial use. Do a Google search for "onion routing". That's the name the Navy gave their concept. Onion routing can be good enough to be practically unbreakable and untraceable. That's why the military wants it. They want to use it for themselves.

Trampy

P.S. On any Windoze machine it's practically impossible to spoof your IP ... but if you're running Linux and have a TCP/IP switch somewhere, your traffic can be traced back to the switch and that's probably a dead end. There are some very good forensic network analysts who would be hard to fool.