Stack-Overflowizer

Redirect any SO clone sites to corresponding original page on SO.

Declutter-Twitter

Filter twitter web interface based on list of keywords. Firefox extension based on bookmarklet (http://www.conroyp.com/2010/03/11/declutter/)

Journaliser

Add a bit of colour to the comments on TheJournal.ie

Twitter-Media-Preview

Add custom media previews to Twitter web interface

DirectToYourDoor

Example of using js geo services and google maps directions to make "find us here!"-type page for small business websites