Privoxy ("Privacy Enhancing Proxy") is a vastly expanded version of Internet Junkbuster.
IJB was, back in the day, a very nice filtering proxy to stop unwanted cookies and banner ads.
However, many other filtering proxies (like Proxomitron for Windows) did their job more flexibly, and worst of all, IJB only supported HTTP version 1.0 - personally, I had a lot of problems getting Mozilla to work with IJB, at least with HTTP 1.1...
I might even use this if Mozilla's own tricks wouldn't work well enough for me... =)
Home page: http://www.privoxy.org/