According to the Ultimate Touring Guide....
The best day to go to Magic Kingdom is Tuesday, the worst day is Thursday.
The best day to go to Epcot is Wednesday, the worst day is Tuesday.
The best day to go to Disney Hollywood Studios is Saturday, the worst day is Friday.
The best day to go to Animal Kingdom is Friday, the worst day is Monday.
The best day to go to the water parks is Saturday, the worst day is Thursday.
They have the complete list
here. (for free)
Edited to add: I think I already mentioned this in this thread, but we went to MK both Tuesday (best day) and Thursday (worst day) and there was a huge difference in crowds!!