Hat tip1 to John Battelle for pointing out Greasemonkey, a Firefox plugin that allows you to run Javascript scripts on arbitrary sites to improve their appearance or usability.
The one script I find most immediately useful is one that fixes some deficiencies of Allmusic. Allmusic is a fantastically useful site, but since it's redesign, it has the annoying trait that every biography is truncated and ends with a Read More link. The script replaces those links with Javascript links that pull in the full biography using XMLHttpRequest. It is remarkably fast considering how sluggish the site often is in general.
1. John must get a lot of hat tips considering he has the number two result on google for hat tip on a page for which the term only appears in links to it.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat