GoBandit

First unit i've seen that actually displays your speed when you download video, Micah posted this video on facebook and he commented;

" Choco the GoBandit had a gps unit in it and when u download to your computer it puts the mask in it so it is embeded in your video very cool. Sorry for my poor camera angle will be better next time'

Not sure how accurate it is. At 20 seconds he unhooks from the harness but keeps accelerating from 67 to 69 kmh until 23 seconds. Seems to be a bit of a lag on the gps readings. There are a number of cameras that do these gps overlays nowadays, a few have accelerometers built in as well.

Yann has a feature in GPSAR that allows you to save a file which overlays on your video for a similar result.