Hi SEOQUAKE is a Mozilla FireFox plugin that displays a variety of web information,

Mainly: (but not limited to)

1) PR rank
2) Alexa ranking
3) amount of links indexed by yahoo
4) amount of links indexed by Google
5) Keyword density checker

What it does is that each time a page is loaded it will display the necessary information on your toolbar.
In addition you can also add a number of other web info to display. I have been using this plugin and have find it very useful

Here is a youtube video on SEOquake
http://www.youtube.com/watch?v=NTpS1ZM1CYk&feature=related

-Hope this helps

Brandon