So I wanted to increase the damage of my Wizard, but wasn't sure which stat gave me the biggest bang for my auction house buck:
Primary stat intelligence?
Basically you enter all your gear stats in, and in real time it calculates the same stats you see in the game.
This way I can 'try on' a piece of gear I am thinking about purchasing from the auction house to see exactly what sort of impact it would have.
I made this tool in about a day, so it may have some bugs. I also only really tested it with Wizard, although it should work fine with the other classes.
The damage calculation isn't exactly perfect. I usually find it off a very tiny amount. I think this is due to the number you see on gear being rounded, but calculations behind the scene in Diablo are done with the non-rounded number. So thus it's a little off on the website, but close enough.
Maybe there is already a tool out there that does this, but I wasn't aware of it.
Oh, there is also a 'Bang for your Buck' box, that will tell you how much extra damage you will get from each stat. Basically, to kinda help you determine which stat you can trade off for another on the AH to get some cheaper gear while still getting same amount of more damage.