For years I've been using 3rd Plan It. While it is handy, I've spend over $600 now trying to keep up with the latest version. My friends and I are having a helluva time sending drawings to each other because we are all on different versions. That and the software is always a bit buggy.
I'm working on a Cotton Gin in HO for my Dixie Central Railroad and I need to draw the structure. 3PI doesn't do plain old drawings very well, so I'm going to switch over the XTrackCAD to give it a try. Lots of people use it.
Download XTrackCAD Software
I'm also going to try Google Sketchup since it is free as well and does 3D drawings.