I recently installed and started using Chickenfoot, a Firefox extension which lets the user write simple scripts that are executed on webapages. The scripts can be triggered automatically or run manually. Either way they manipulate the webpage after it loads.

Chickenfoot is similar to Greasemonkey except that Greasemonkey forces you to use properly formed Javascript and to actually understand the elements of the DOM to manipulate the page while Chickenfoot helps you along. It comes out of an MIT research project on Computer-Human Interaction so it recognizes common form elements and provides shortcut commands to manipulate them.