FYI, it is not free.
http://www.lexaloffle.com/voxatron.php
http://www.lexaloffle.com/bbs/?tid=1336
v0.2.4 is the first update that exposes many features of the second (and final!) generation of the Voxatron engine. It introduces new tools for designing simple monsters and pickups, which all run on a new physics engine. I've retro-fitted the old monsters to work roughly as they did before, but you might notice a few differences -- for example, monsters jumping on each other, or carrying fruit around on their heads.