Google SketchUp is a free, 3-D modeling tool that can be very useful for planning and building multi-touch tables. SketchUp allows you to work with precision in measurements to create scale models, but is simpler to use than most CAD applications and allows for a diverse range of model complexity.
A Pro version of SketchUp also exists, which can be purchased for $495 USD. SketchUp Pro is more feature-rich than its free counterpart, and includes a tool to make your models into blueprints - LayOut. Information on SketchUp Pro can be found here.
If you have never modeled in 3D before, SketchUp is an excellent place to start, and will prove quite useful in determining measurements and creating a plan for your table. Here are some good places to get started:
Official SketchUp Training Videos
Architectionary Video Tutorials
3D Warehouse - an existing repository of user-created models
Post your multi-touch models, questions, advice, and SketchUp resources here!