I've been playing with the 'free' version of Google Sketchup. It takes about an hour or two to figure out, but its perfect for creating buildings and such. Unfortunately, the free version does not allow file export (though you can save your work).
But, I found out there is an application called "Flux Studio" that will open the Sketchup files, to which you can then export them as .obj files (though not sure how ethical this is - though Flux Studio appears to be a fully legitimate application affiliated with Sketchup). Then you can use PoseRay to convert the .obj files to TGTP ready format, and then import them to TGTP.
For even more flexibility, I've been creating complex 'parts' in Sketchup, and then using Vue 6 infinite to easily assemble these parts into working structures. Here is a castle I just created for use in one of the images I am rendering...