Earlier this week Yahoo! released a public version of an internal website performance tool called YSlow. Visit the YSlow page for instruction and downloads. YSlow runs on Firefox and requires the Firebug extension.
I’ve used YSlow a lot to help improve the Messenger web sites, so I’m pretty familiar with its operation. I can see where some people using it on either personal blogs or smaller sites will be surprised a the somewhat harsh grading for things like using a CDN. For a large site with huge traffic like Yahoo, it’s a no-brainer, but for a small blog it doesn’t make much sense. (See also Paul Stamatiou’s review for more of an explanation on this.)
That being said, I think there is value for developers to run the tool and gauge their site against the rules for high-performance web pages. YSlow provides some great detailed screens that will show you which of your content is gzipped and cached effectively, versus that which is not.
I ran the tool on my own site and initially got a grade letter D! I made one quick change to disable ETags which raised me up to a C. See the screenshot below for an example.
(Via Jeremy Zawodney)
There's a very similar tool at http://linuxbox.co.uk/website_performance_test.php that I often use. Not as convenient as a plugin, but I think it does a few things the Yahoo plugin doesn't