Sandboxie will isolate the outcome of any website you visit, including any software or script installations, which can then be undone.

I remember first hearing about flock. I actually got a version of it when it was just skinned to look different than FireFox. I didn't expect the project to get this far. Give it a wirl, looks to be promising.

Flock 0.9 lands gracefully - The Flock project has been building a "social Web browser" since 2005. The upcoming Flock 0.9 release adds new blogging features, integrates media streams into the browser, and includes an overhaul of the Flock bookmark system. It's not perfect yet, but Flock 0.9 is a big leap forward. [Linux.com]