http://freeware.intrastar.net/cadsoftware.htm
Here's a few to choose from.(Google is your friend)
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.