Google Skymap and Goggles. Both almost totally useless but a neat trick.
CAller ID Faker wows everyone when I show them. I call them from their own phone number and select the male voice option(doesnt sound like me at all). Gets tons of attention and laughs.
Using my phone as a wireless mouse and keyboard to control any computer in my house. Best part is it's free! download gmote from the market then install gmote server on comouter (s) and it's a remote!