Racelogic PerformanceBox is a datalogger that measures lateral g's, acceleration, lap times, and much more, you can graph everything. I find this very useful at track days because you can see where on the track you are fast. Also the unit is self contained and only needs to be plugged into a cig lighter outlet and then drive. You can easily throw it in a friends car and gather some data from there laps to compare. $499 might be more than you want to spend, but it is probably a 5 to 10 year investment, similar to a Valentine 1.
PerformanceBox and DriftBox from VBOX USA
-David