Fuel Pressure Gauge
I have a scanmaster unit, and a few gauges: hood-mounted fuel pressure, boost (0-30 PSI), water temp, and oil pressure. That's plenty of information for me to tune with at the track. I've heard that an EGT (Exhaust Gas Temperature) gauge is also good to have, but I don't have that yet.
As far as the fuel pressure gauge goes, some guys install them at the end of the fuel rail (at the schrader valve location). to check while under the hood.
HTH!