I have this one:http://www.dod-tec.com/product.php?cat_1st=4&cat_2nd=6
I bought a pre-production one as a sample which has some bugs, but I think they have been resolved.
It is nice and small but takes a fairly good quality video, as good as any similarly priced dash camera.
Uses a micro SD card
Overlays GPS speed on to video
Breaks up files into short 1 min clips to avoid data loss when power is disconnected
Starts recording automatically
If you want something of even higher quality, then look into something like the Contour HD cameras. They're designed for recording sports like biking, skiing, etc.. but offer pretty good quality recording. No speed overlay, but they have a GPS model which records the GPS track including speed.