YSlow: Yahoo! Performance Evaluation Tool

Yahoo! has released a new Firefox extension called YSlow that plugs into Firebug and gives you additional performance measurements and suggestions for improvement. It measures things like how many HTTP requests your pages requires, whether content is gzipped, if JavaScript files have been minimised and so on. It will give you a quick "grade" for a page to let you judge how many optimisations still need to be performed (my homepage scored a 74 – C; not too shabby).

Even this is not yet a final release it looks extremely promising and is already quite useful as part of the "Firebug suite". I highly recommend checking it out.