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.
The software and most of its plug ins are licensed under the Eclipse Public License which allows anyone to download the source code, study it and make changes on it without permission from Archimedes' team.