I gotta agree with Nubus for distortion you can't go wrong with a Proco Rat. I used to have a Turbo Rat which I'm kicking myself for letting go. It had the perfect kind of thick distortion good for alternative to whacked out.
If not I would suggest the Digitech Bad Monkey which is a like a more modern sounding Rat with smudge less output.
http://www.digitech.com/products/BadMonkey.htm
