Greasemonkey

I am sure many of us heard a lot about the terminology Greasemonkey but did not understand what it is all about.

Greasemonkey is an add-on onto Firefox. There are reports of it working even on Opera, Safari, IE to limited extent though. This is the master add-on which is like a container for all the 'User-scripts' which gets installed into Greasemonkey.
User-scripts are javascripts (extended-AJAX too) created to customize look, feel and functionalities of a webpage.
There are thousands of existing User-scripts available at the official website of User-scripts
Greasespot is Greasemonkey's homepage

Popular list of user-scripts:
by online-tech-tips
by Firefox facts
by Life-hacker