Realcalc
I've tried 'em all, this is the BEST calculator on the market. Also does conversions. Easy to use buttons etc. Looks like a real calc when you have it on the screen.
Google Sky Map. An absolute MUST HAVE app. Hold your phone up like a bar of soap, and it shows you the sky. It works like magic.
Tricorder. As in a Star TRek Tricorder. Makes the use of about every sensor in the phone. Very cool app, and has actual use such as an audio db meter.
Mr. Number Caller ID. You agree to give it permissions to use your phone's contact list. It reaches into a zillion other user's phones to root up the incoming number. Makes 411 services look pathetic, and just about produces all info on who calls even if it's a telemarketer with obscure numbers.