Exhaust Gas Temperature gauge.

It allows you to see how hot the exhaust exiting your engine is. Indicates if you're running lean/rich i.e. the hotter the temps, the leaner you're running and vice versa.

Also great if you're running antilag to monitor your temps.

Anyone who does track work should run one, so you can see if you're exhaust temps are getting unsafe....extremely high temps can ruin valves and turbos.