Pro-Ject Strobe It Disc and Alignment Tool
This stroboscope disc allows you to accurately adjust the speed of your turntable to 33½, 45 and 78rpm. Designed for use in artificial light, the disc's pattern will appear to be static when the turntable's speed is correct (due to the 50Hz mains frequency).
All you need to do it place the disc on your platter, turn on your light, and then adjust your turntable's speed control until the pattern on the disc doesn't move. As an extra bonus, the rear of the disc features a handy diagram for cartridge alignment.