After having posted the DH Sharpshooter calculator I thought I'd share another calculator I've stumbled upon which is extremely useful (I might make a compilation thread of these).
Credit for this one goes to Zakarumite of the Diablofans forum.
Diablo 3 Gem Calculator
I've been using it to see where there are opportunities to make gold from combining gems whenever i have lots of Tomes of Secrets lying around.
Enjoy.