Greasemonkey is a great tool for extending your browser...if you're a hardcore programmer. Chickenfoot is an alternative end-user programming environment, embedded in Firefox, developed by Michael Bolin, Robert Miller, and crew of M.I.T's User Interface Design Group. The major wins of Chickenfoot are a focus on naive programmers, an interactive development environment, and a high level representation of the rendered Web page as the major object of manipulation.
[Via Lambda the Ultimate]