Whats GreaseMonkey ?
Its not a simple browser plugin ! it has more to it..
GreaseMonkey is a plugin, which can execute scripts (called user scripts - user scripts are like java scripts). There are many uses of these user scripts :) like
1) You can use script named "xyz" to block all ads from Google
2) You can have your own CSS, Fonts overriding/overloading that of the web site's fonts/css
Grease Monkey is orginated from a concept called Active Browsing, You can find the script repository at http://userscripts.org/