While at it, i think this new upcoming modeler called Void World may be worth a mentioning. The current homepage is horrible, and it is not ready developed yet. But the software itself goes somewhere into the direction of Silo, with the benefit that this software here gets still developed.
You can grab the newest beta build in this thread here: http://www.polycount.com/forum/showthread.php?t=74005
Edited by tiles, 10 July 2012 - 02:14 AM.