A d6 based on an elongated
Dürer's solid.
This shape is an elongated, truncated rhombohedron which has six pentagonal faces and two very small triagonal faces.
The idea is that the die is unlikely to land on the small triangles so it can be used as a d6.
This die is good for those who are into mathematical oddities (and think that a die based on the Durer's solid is just a "pretty neat" idea) or for use in a game where a die which lands on 1 through 6 is needed most of the time, but where it occasionally landing on another side could bring another rarer result. (The remaining two triangular sides are unmarked and can be used for any special purpose or can mean "roll again").
I will be the first to admit that this shape is not strictly an isohedron, and thus is not entirely suitable for use as a die, however, for the six numbered sides it is equally likely to land on any of them. Basically, I just thought it would be "pretty neat" to make a die based on Durers solid and it appears as if no one else did, so I gave it a shot.
(Note: that this shape has been elongated beyond the way it appears in the original painting Melencolia so that it will be even less likely to land on the triagonal faces, which are now equilateral. In other words, although according to the Wikipedia article I've linked to above, scholars believe the angle to be between 72 and 80 degrees, but in this die the angle is about 60).