FCEUX - SMB "Ghost", among other things
Thread about this on TASvideos
YouTube video (features a lot of debug output, though)
Some images:
%20(PRG0)%20[!]-30.png "4P SMB1? Well, no, but...")
%20(PRG0)%20[!]-31.png "Not first any more...")
And if 3 replays isn't enough:
%20(PRG0)%20[!]-32.png "You could theoretically do many, many more.")
So while this may not be quite on Sunday (yes, I decided Sunday was better), well... now you know why :)
It isn't quite finished yet, but most of the basic functionality is done. Hopefully I can be finished with this soon :D