The android OS has a very basic calculator built in, but it's pretty lacking. I really like this one, which has all the normal scientific calculator features you'd expect, a good history display, and puts commas after every 3 digits, like a good little calculator.
No ads, no nags. There is a paid version for $1 that adds some features that frankly I don't miss. But it's probably worth supporting high quality work like this one.