Page Speed release by Google

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.

Page Speed performs several tests on a site's web server configuration and front-end code. These tests are based on a set of best practices known to enhance web page performance. Webmasters who run Page Speed on their pages get a set of scores for each page, as well as helpful suggestions on how to improve its performance.

Sounds like the ySlow addin released by Yahoo.

Link:
http://code.google.com/speed/page-speed/