I like
AutoRealm for laying out blueprint style maps. You really have to take more time to work with it for nicer maps, but you can do it.
I have also used
Dungeon Crafter (version 1) and some of the add-on tilesets to some success. There are a couple of "Sci Fi" tilesets that can work okay. When you start to mix and match with generic tiles from other sets, you can sometimes get away with more than you think.
Those are the only two that I have any experience with, though. If all else fails, you may be able to search
Sourceforge for some decent and easy CAD programs.