A regular tetrahedron (better known as a three-sided pyramid) with three numbers on each face. This odd die is best thrown by tossing it a few inches into the air with something of a spin to it; when it lands, the number on each face that's right-side up (on most d4s the numbers will be on the bottom edge, though on some they're found on the top point instead) is the result. Naturally, on the vast majority of d4s, the unmodified result of such a roll is a natural number from 1 to 4. In the various incarnations of D&D and AD&D, common uses for d4s include wizard hit dice and dagger damage dice.

Don't sit on one of these. Especially not a razor-edged one.