The metronome is probably the most useful non-drum item that a drummer can own. After all, you need to learn to play good time and a metronome can help you do that. A metronome is simply a device that produces a click at regular intervals so that you know where the pulse is. Metronomes used to be mechanical, keeping time with a pendulum that rocked back and forth. Nowadays they’re electronic and come in a variety of styles that can range in price from about $10 to as much as $175 (retail). They all keep good time but vary in the amenities such as headphone jacks (a must-have for drummers) and the ability to subdivide the measure.
I recommend getting a metronome that has a headphone jack. If you can afford it, I also recommend getting a metronome that can subdivide the measure into eighth notes, triplets, and sixteenth notes as well as being able to produce a different tone on the first beat of a measure. I’ve been using a Boss Dr. Beat for 20 years and it still works perfectly, so a good metronome lasts a long time if you take care of it.