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.