Basically a glorified timing device hooked to your iPhone's inner space/time continuum counter gyro or whatever it is in there that can determine how fast the phone is moving. I downloaded this one for the heavy hit of $12.99 and just barely played with it today, I'll try more tomorrow. You can do measured 0-60 runs and it also figures out max speed, max acceleration in G's, average speed, and ET's for 60', 330', 1/8 mile, 1000' and 1/4 mile. Starts to sound kinda fun huh? It crunches it all and turns it into a graph as well as a list. Of course, you can save the runs, name them, average them, blah, blah. I haven't figured out how to watch for the green light and then slip it back in my pocket, but I imagine you could zip tie it to the risers or something and just take off once the green lights up. You could always mount one of these attractive old-school mounts... Top speed could prove fun. Think you are hauling ass on that stripped down chopper with no speedo? Wonder how fast you are going down the freeway on average? Could be kind of handy. The estimated horsepower thing seems closer to horse shit, but who knows?
SOPODS Speedometer 1.1
Now this one is only $1.99 which is cool but only works on the newer 3G models. Wish I would have read that fine print before I paid the two bucks. Looks like a super simple speedo, which could prove useful at times. When I was messing with gearing on my Sportster I borrowed a GPS and did a couple freeway runs looking at the indicated speed and then checked it again after going up a couple teeth. This could do the same thing, but using a device you already own.
Now instead of a sight gauge on your gas tank, recess a little divot for your iPhone to sit into. Just kidding...