YSlow

It's not only once that you have felt that a website is running extremely slow and it's not because of your connection.

Now and thanks to Yslow, this add-on for Firefox you'll know what happens according to Yahoo's rules for high performance rules.

Finally, if you own a website, it will be really useful if you want to make it faster, because it will tell you all your mistakes and you'll be able to solve them.