CoreWin 2.2 is Chip Wendell's feature packed Mars for Windows.
pyCorewar is Jens Gutzeit's Python Redcode simulator.
fMars (fast Mars) is Michal Janeczek's Redcode simulator.
A.R.E.S. is a simulator for Windows with extensions allowing stacks, absolute addressing and console i/o. Includes an operating system written in Redcode.
nMars by Pavel Šavara
Phil Knight has written two scheduling utilities for Windows, CoreWar Tournament and TestWarrior. [archived locally]