Six sided die with ball bearings (1-6) on each side. With a .25mm clearance around the balls they should all move freely. Slightly smaller version, by request, but I couldn't make it much smaller without getting a little too close to the minimum space needed for moving parts. They're wedged in there pretty tight so you'll need to take some time and break them loose.
Six sided die with each side made from a gear with a number sitting in the middle. Each gear is attached to an axis inside the die. Recently added diagonal sticks to discourage it from landing on a corner.
Another die made with a gear for each face and the number inside that gear. In this one the gears are attached to each other by intersecting teeth and they cannot move, but it still looks cool if you ask me.
Know someone who's a 20? Keep a little (1.5 cm by 1 cm) photo of them inside this 20 sided die necklace. Comes in two parts. Pass your chord through the hole in the top half, through the loop inside the bottom half, and back out through the hole again. The loop is attached to a platform where you can glue picture.