Not free, but saves you over buying kits. Evans Design Model Builder You design buildings and print them out on paper to be attached to a more solid material.
Then there's Kit-O-Mat in which you design a structure using their software then they create the laser-cut pieces for you to build it.
Chip
Building the Rock Ridge Railroad with the slowest construction crew west of the Pecos.
http://freeware.intrastar.net/cadsoftware.htm
Here's a few to choose from.(Google is your friend)