Forgive me if this has been talked about, but this sim is one of the simplest to use that I've seen. The EJ compendium author says it is his favorite right now. I can see why. I really hope the makers of the site extend it to other classes soon for my alts. It is limited to DK, Shammy, and Druid right now.
Enjoy:
http://www.askmrrobot.com/WowSimulator.aspx