TaurusVM

TaurusVM is a virtual machine with esoteric properties (well, technically, a collection of VMs). It is managed by the Zodiac Working Group.

Virtual Machines
TaurusVM supports multiple Virtual Machines with different esoteric properties. An optimal [lang]->TVM compiler will analyze the program for the optimal VM to use, but any real compiler is not expected to. As such, a compiler can just use IndeterminantVM (or another, if they please, but that one should be the easiest to work with usually). Speaking of which...

IndeterminantVM
IVM supports variable (indeterminant)-length arguments. Instead of ADD, it has SUM, et cetera.

ThueVM
ThVM is based on string operations. It is not actually that much like Thue otherwise.

ArbourVM
AVM is like a normal VM, but the Registries lie on a tree as opposed to an array. It uses a pointer for simplicity.

GreekVM
GkVM is a kind of simulation of what the Greeks' (who did a LOT of geometry) computers' ISAs would look like. The Registries are 64-bits, with the first 32 representing a point's position on the x-axis and the second 32 on the y (in floating-point, of course). Operations involve constructing things out of these points.

External resources

 * The Homepage