A very common feature on manual transmission race cars (or street cars used for racing, or street cars that want to look like they're used for racing)
A shift light is a very bright light typically mounted either on the steering column or just above the gauge cluster. It comes on at a predetermined (usually programmable) RPM to alert the driver that the car is approaching its redline.
The gap between the shift light's actuation RPM and the car's redline is a matter of tuning, and depends on how quickly the engine revs. The idea is to have the light go on perhaps one second before redline, so the driver has time to shift.
Now, you might think that race car drivers wouldn't need some stupid light to tell them when to shift. That's true, especially in the higher gears where it takes some time to get up to the top of the gear.
However, in many race cars, first gear goes by in 2 or 3 seconds. It's very hard to shift at the optimum point (too early, and you miss some of the powerband; too late, and you bang the rev limiter, which slows you down and can upset the car in cornering situations).
A good shift light is bright enough to be seen in the day, without being so bright that it scares or blinds the driver at night. High output LED's are increasingly common for this application, typically in red.