Basically an HTML filter. You can filter ANYTHING on a page. Frames, tables, anything that's actually present in the HTML source is editible here. I could've changed the little falling white things to smiley faces if I wanted to, just by looking for that "peace.gif" instance and replacing it with a smiley URL.

http://www.proxomitron.info/files/index.shtml

Read a bit of the site before even considering downloading.