Coolest thing I've done on Android is use it as a virtual dashboard for my car (speedometer, tachometer, boost gauge, reset cel, etc.). Torque app + OBDII Bluetooth sensor ftw!
A close second was using my RAZR while going out to chop a Christmas tree. I used the GPS (MyTracks app) to see where we'd been and I would mark locations on the map where there were trees we liked and take a photo of them. Pretty awesome way to not get lost and to not have to say, "where was that one tree that we saw 20 minutes ago that looked good?" Trees that you initially passed over as "not good enough" start to look better the longer you've been out in 15° weather.
I also use my Androids as ereader, camera, Bible, alarm clock, media player, communication (cell, text, im, VOIP, video chat, etc.), GPS (navigation, hiking, etc.), stargazing, real-time price comparisons while shopping, note taker, shopping lists, reminder, HIIT timer; tons of other stuff I can't even remember. Love Android!
Sliced from my RAZR