Design and Navigation Software - Open Source, Freeware, and Shareware
OpenCPN is a free software (GPL) project to create navigation software for use under way or as a planning tool. OpenCPN is developed by a team of active sailors using real world conditions for program testing and refinement. It is fully cross-platform and can be used on Linux, Mac and Windows.
You can purchase QCad Professional (recommended) and the QCad book at: RibbonSoft, GmbH. If you use Linux you can install the GPL version from your distros package manager. It is not as full featured as Qcad Prof, but for reading .dxf files and light CAD work it's OK.
There is a free foil analysis program (using the Vortex Lattice Method) here for analysing single surface foils, and so it is of interest to sail designers. Unlike Designfoil, it can accept multiple surfaces, so can be used for ketches/schooners/side-by-side rigs.
You can get the Designfoil foil analysis program, with an easy-to-use graphical interface, from http://www.dreesecode.com/ . Use it to study the aerodynamics of sail cross-sections and planforms. The demo version is free and can be used for an unlimited time, but is limited in some aspects of storing and exporting your work. This is Windows only but it does work under wine for Linux users and of course can be run in a virtual machine or PC emulator.