Porsche has one, which also has a G meter, so kinda fun, but says I'm doing 121 MPH...and Speedo says 80 (which is correct, I wish it was 121 MPH!)

Any apps that work?

I'm testing props, so while I'm testing it would be nice to see some numbers. Don't want to use it for normal driving, the gaffrig is fine for that.

I use "speed" takes a minute or so to be accurate but once running it's great.

http://itunes.apple.com/us/app/speed/id286756404?mt=8

a friend of mine wrote SpeedPro and Speed Freak (in iTunes.) The latter allowing 2 or more users to watch eachother's course speed simultaneously. The company is Mother Tucker LLC. I used Speedpro all the time and it works great.

"Navionics" is hands down the best app I've found. It's like $10 & covers every waterway in the country.

+1 Great App!

JustSpeed has worked fine for me as a simple GPS speedo.

For navigation I have the Navionics. There are some holes on lakes around me but for the most part they are very good. For Speed/GPS I use Speedometer by Sprotstracklive. It will show you your speed at different intervals while laying down a bread crumb trail over the map. It is nice because you can just drive then look down later and see what you were doing.