OpenVSP is a parametric aircraft geometry tool. OpenVSP allows the user to create a 3D model of an aircraft defined by common engineering parameters.
Compared to anything else I've tried, OpenVSP is blazingly fast, and * taa-daa * it's free.
Here is an example design that comes with the program showing its capabilities:
Here are several doodles I produced in a few minutes each:
Here's my x-plane that I've been using to familiarize myself with the program's drawing tools:
There is a very good manual for OpenVSP which includes instructions on all the basics and a few ok video tutorials. To get beyond the basics takes some dedication; I'm trying to figure out at the moment how to construct a typical aircraft coss-section which inludes a canopy. Hopefully, there will be some online discussion developing around the program which will help to answer questions. Since NASA has released the program as Open Source, perhaps there will also be some development of additional features and import/export options.