In M.U.G.E.N, The Atom has been made once by AngelusEcks and appears in a hyper of Green Arrow.
This version of The Atom is based off of Ray Palmer version, although the Ryan Choi version makes a cameo in one of his intros. He uses custom voice clips from an unknown voice actor. He has several special moves such as the "Mite Punch" which is a pretty generic dash punch move that deals a fair amount of damage. He also has a few super moves, which Ryan Choi appears in as an assistant.