I dont think this is a legal device,but as an extra bit of kit it is very nice...
http://www.vi-performance.com/homepage.aspx"Monitor various parameters like speed, revs, engine load, water temperatures and pressures as well as many more* on 3 individual easy to read gauges."
Requires ODB-II compatibility,which may be limiting?
Measures lateral G force aswell,cool!!

But then I am a bit of a geek!!