Netbeans 6.x and Firefox+Firebug
In the past I used Aptana Standalone or as a Eclipse plugin. The Pro version has some nice addons like the embedded IE (to the always embedded Firefox) and debugging support for both browsers.
I found after hours of testing, that Suns Netbeans is the best PHP and Python IDE for Windows and Linux. I was surprised that also Javascript support can hold the candle to Aptana.
So Netbeans is my recommendation (not only for Javascript).
Give it a try, its free!
