Audio Damage has a great multi-band distortion plugin called Kombinat.

For sick and extreme effects... Decapitator! (I love this plugin)

http://www.soundtoys.com/products/Decapitator/

For specific tasks... Speakerphone..

http://www.audioease.com/Pages/Speakerphone/speakerphone.html

Also, plugins like Guitar Rig, Amplitube and GTR have lots of interesting distortion/overdrive components, plus a ton of invaluable tools for sound design.

I have reached for the SansAmp PSA-1 many, many times. Great, versatile little plug-in.

SansAmp, Massey TapeHead, and Waves L1 - seriously, push that puppy hard to get a very unique distortion effect!

Something you'll probably find from me, a lot, is free plug-ins.

I use Atomsplitter's Destroyr almost exclusively for OD/Distortion... The only times it hasn't been as effective as I've wanted it, I either wanted more bitcrushing (which I use CMT Bitcrusher) or when I want a smoother overdrive, which I'll use just about any amp simulator for.

I have really come to like the amp simulater that is in Nuendo 4. I've found it versatile, and usefull when I want to dirty something up. I almost always still mix in the original clean track with the distorted one. Depends on the situation of course.

I'm limited in third party plug-ins (I only have three), therefore I have to find creative ways to work with what I got. I find most of the fun in sound design comes from that.

ohmicide!!!

absolutely wicked distortion, multi-band (4 bands), tons of interesting filters, endless mutations...

I love Isotope Trash. very useful features. Pre COmpressor, pre-eq, post eq, tons of distortions, cabinet modeling, and some bells and whistles - delays and such (the analog delay is nice and thick).