Huge fan of Clockwork Empires - especially CE, for which I am in Early Access. I link to a preview of an indie game I am playing, kind of a Roguelike Dungeon Keeper. It is in Alpha, but cool. Preview - http://matthewhopkinsnews.com/?p=653 Official Site - http://keeperrl.com/
Also, not to tease the CE Developers but KeeperRL has slavery without any difficulty. You can get prisoners and force them to labour.