I know there are at least a few Robotron shareware clones that exist out there, and I thought that people may be interested to be informed of those games: which platform(s) the game is for, how much it costs to register it, and, of course, a link to the website where the game can be found. So here we go... feel free to add to the list!

Ultratron by Puppy Games, 6MB, $10. For Mac, Windows and Linux. There's also an older version of the game, called Puppytron.

Kill Monty, by Freeverse Software, 20MB, $12.95. For Mac OSX (10.2.8 or later) only.