turn your DROID into a full qwerty keyboard, and the touch screen becomes a mouse. tap, double tap, everythign... it connects to your computer though Wifi, and becomes a full control surface. you also can preview videos and photos on your desktop/laptop on your phone.
CamCalc - FOV, Focal Length Equiv., Color temp conversion, Solar calc.
Weatherbug
Stopwatch
SunriseSunset
SL Digislate
Ulysse Gizmo - (One of my favorites)
Google goggles
Google Sky
LOVE Vlingo, it's probably the one I cannot do without.
Shazam is very cool.
Justin.TV is a cool app. It'll let you wirelessly stream from your phone to your Justin.TV account. Came in handy during my sister in laws surprise party. I streamed and my sister in Cali was able to watch the whole thing from her laptop.
Dropbox
Carr Matey has helped me not lose my car a couple of times.
Oh, and if you like to hike and/or geocash c:geo is seriously awesome!