"Dead pages, broken links, the scourge of the internet. Powerhouse sites like Slashdot and Digg can bring a server to its knees. What do we do when a page is dead but we still want to see it? Call in the clerics, and perform a resurrection..."
With this add-on, you can highlight a name on any web page and find information on that person on a variety of different sites. Makes professional reading more effective and informative.
"Save Money. Save the Environment. Tired of printing web pages only to find your printout is full of ads, empty space and other junk you don't want? PrintWhatYouLike is a free online editor that lets you format any web page for printing in seconds!"
I use a firefox addon called Aardvark and can vouch for the advantage of being able to trim your webpages on the fly, especially when removing advertising and other junk before preparing a page for students, keeping it child friendly.