The Google Calculator has been a big hit with so many sites writing reviews about it and I'm sure many, if not all have tried it out. You can search for 2+2 and google will return 4. I was impressed simply because it adds in the brackets for the order of operations but I never really realized the power of this tool until just now when I searched for cm to inches and gallons to liters.
I'm a bit slow to the party but a very sweet addition.