Being the slow kid on the block, I just started dinking around with Greasemonkey. I wrote the obligatory google video and youtube FLV exposers and put them in my greasemonkey script archive (basically, they just make the title of the flick an href so you can dl it.

Greasemonkey could use an include directive, so that libraries of JS code would actually work (as opposed to GM_xmlhttpRequest'ing the libs and eval'ing them). Of course, you'd have to write those libaries because doesn't have a social ranking of the scripts (just tagging), which makes finding anything an exercise in Voynichian translation.