Archimedes is a free and open source CAD (Computer Aided Design) software. It is a robust system built over Eclipse’s Rich Client Platform fully based on plug ins. This means it is easily extensible but also has a pretty thin core that allows you to select the features you need and drop the other ones easily.
Archimedes works on GNU/Linux, Mac OS X and Windows and it only requires you have a Java Virtual Machine installed which is also a free software. This means that anything generated with Archimedes is easily recoverable and reused.