Google Page Speed - a Firefox plugin that extend Firefox is cool.

However top recommendation for Javascript on one of our sites....

Minify JavaScript
There is 44kB worth of JavaScript. Minifying could save 6.3kB (14.3% reduction).

    * Minifying https://ssl.google-analytics.com/urchin.js could save 3.7kB (16.7% reduction). See minified version.