If your amp has an effects loop, I can recommend the DOD Bad Monkey for distortion. I don't usually like DOD stuff, but the Bad Monkey has a wide range of tones in it, and when its on, it sounds really good.

The reason I returned mine, is that it doesn't have true-bypass switching. When you turn it off, the signal still goes through part of the circuit, and it sucks all the tone out of your axe. :p An effects loop would get around this problem though, so I can happily recommend it for use with an effects loop or some other A/B switching system that will bypass the pedal when its not being used.