Sean Willson has a great collection of favelets (or bookmarklets) which is basically “a bit of javascript code that you can put in your bookmarks to perform some action in your web browser.” Sean’s favelets range from browser resizing to disabling stylesheets to performing searches to validating HTML and CSS.
Also worth checking out is some favelets for the Wayback Machine courtesy of the Kalsey Consulting Group. Also there are move over at About.com, Bookmarklets.com, Google Directory and Jesse’s Bookmarklets Site (highly recommended!).