I hereby retract any complaints I may have recently made about paying taxes.
The heroes at NASA just gave us some open source aircraft design software called Open VSP (vehicle sketch pad). This open source software is designed to let the user rapidly create high-fidelity parametric design/structural layout for conceptual aircraft designs, resulting in models that can be processed into formats suitable for engineering analysis. According to the wiki, it's been under development since the early 90's, and J. R. Gloudemans, P.C. Davis, and P.A. Gelhausen presented a publication on the development of VSP at the 34th Aerospace Sciences Meeting and Exhibit in January 1996.
I'm guessing VSP has come a long way since then.
This ten-minute video introduces some of the features of Open VSP, by building and analysing an SR-71 Blackbird-like model:
The above video was created by Bill Fredericks, and stolen from the Open VSP Video Tutorial Page, which also contains a handful of more in-depth video tutorial videos created by Ami Patel.
The Open VSP code repository is on GitHub. The Open VSP wiki is here, and the Open VSP google group is here.
If you want to get going with some open source aircraft design, click here to visit the Open VSP project website and download the software.
You can find lots of free resources for open source aircraft design on my aircraft design page.