I use google
Besides that, there's the one built into Windows, Mac, Linux, or whatever you happen to be running.
More info (what functions it needs to have, what OS you are running, etc) is needed to give you any type of better answer
lol whoops, wasn't paying attention (I really should get more sleep....)
The question still remains of whether or not it needs any special features.
I have no personal experience (I'm more of an android guy), but this list looks pretty promising
http://www.ilovefreesoftware.com/16/ipad-2/free-ipad-calculator-apps.html