Page Speed is an open-source Firefox/Firebug Add-on. Webmasters and web developers can use Page Speed to evaluate the performance of their web pages and to get suggestions on how to improve them.

Fantastically useful Firefox plugin to help avoid blindness.

RollTube is a Rick Roll Firefox Extension that switches every YouTube video with the infamous Rick Roll video. The experience has been described as “like the whole internet ganging up and rick rolling you at the same time”.

The Action Streams plugin is an amazing new plugin for Movable Type 4.1 that lets you aggregate, control, and share your actions around the web as well as a list of your profiles on various services.

Aperture to Picasa Web Albums is an Aperture plug-in that automates the process of uploading photos from Aperture to your Picasa web albums.

BlogMate is a free, graphical plug-in for the popular TextMate editor that allows creating and editing blog posts for MetaWeblog-enabled blogs from a floating palette within TextMate.

Now free; I've been trying it for the past few days and it's pretty useful.

Must-have.