A Java-based filtering proxy, known to work in both *NIX and Windows.
Muffin does everything Internet Junkbuster doesn't do, but it can also be made to block stuff more aggressively based on content, and modify content too (Junkbuster merely passes through what it is allowed to and puts in a dummy for something that was not not allowed). It is sort of like the equivalent of Proxomitron: It stops looping animations, removes Java applet code, blocks images with certain sizes, shoves background music where it came, and more, everything you need.
Like Proxomitron, it's very configurable and even has configuration GUI. (Though I have to admit I'm still looking for the actual configuration. =( )
Home page, as of writing: http://muffin.doit.org/