Im a big fan of JQuery. Its a truly versatile javascript library that greatly simplifies writing javascript code,and also one need not worry about cross-browser issues, which was the bane of plain old javascript.
Recently i came across an interesting bookmarklet written by Karl Swedberg, called JQuerify. It is a small snippet of javascript code that we can bookmark and use to embed jquery support on any page we are currently viewing in the browser.
Really useful for playing around with JQuery in the Firebug console. Go, give it a shot !