I have the ScanGauge, probably the best part about it is that I can add 4 additional gauges in real time as I drive.
MAP, MPG, GPH & RPM usually, but there is a choice of more than a dozen.
The Manifold absolute pressure is in fact a measure of turbo boost.
It also has trip economy and distance to empty functions that I don't use, mostly because I don't record the exact amount of fuel I get at every fill-up.
Battery voltage has been interesting, it seems to be more a function of temperature than electrical load, i.e. the voltage regulator provides higher voltage to the battery when the temperature is low. NO, this isn't because the battery is cold and has a higher internal resistance - I did a little experiment to check for that (-: