Greasemonkey

Firefox extension allowing a JavaScript to be "mixed" into a browsers page. Provides a very effective means of making quick browser based hacks on sites outside of your own control.

Greasemonkey scripts can eventually be turned into full fledged Firefox extensions to allow others easier access to installing your script (without the need for other people to first install Greasemonkey).

Install page
Greasemonkey Hacks Mark Pilgim's collection of useful hacks with GM
Information about cross-browser scripting]
https://arantius.com/misc/greasemonkey/script-compiler.php
Metadata block
RegExp

Example Projects

Either GM scripts (or possible to have been prototyping as a GM script)

Timopedia
Pirates of the Amazon began as a humble Greasemonkey script
as did Linda Hilflings Google Misspelling Generator
FAT's Google Alarm is a proper Browser extension, but could have been prototyping as a GM script
http://addons.mozilla.org/en-US/firefox/addon/MurdochAlert-details/