I have started to dive into MooTools and Greasemonkey. I am interested in how the DOM of Firefox, allows users to re-program the content in web sites without actually modifying the source code of the site. This method can come in very handy when building or debugging sites.
if you already have Greasemonkey you probably already know about http://userscripts.org/ which is the place where you can find lots of scripts to make your web browsing life more convenient.
I was told about this amazing library of plug-ins on Cnet.com that could help you develop some cool Apps. if you are the coder/proggramer type. This is the address to the gold: http://clientside.cnet.com/js