If your phone didn't already come with it - google maps is pretty dang useful.
I recently downloaded shapewriter, which I like a lot. Just drag your finger around the keyboard to type text. Takes a little getting use to, but works great.
Google sky maps is pretty cool if you want to know what stars, planets, constellations you're looking at.
scanner radio is kinda cool - you can listen to any agencies' scanner in the country.
Pandora Radio.