Switchy

Switchy is an esoteric programming language created by User:Poolala, which changes an instruction's operation every program cycle.

The program stack is also reversed every cycle.

There is a binary variable called flip, which starts out at 0, and flips its value every cycle.

Finite State Automaton
Switchy is not Turing complete and acts as a Finite-state automaton. Note: because none of the states result in a stack of size two or more, a # can not be used.

Implementation
An implementation in Ruby by User:OriginalOldMan can be found on Talk:Switchy.