Archimedes is a computer aided design (CAD) solution focusing on architectural design. While the current version of Archimedes may not be mature enough for all - this project is very promising.
It already has many of the most common and advance drawing features you will find in a commercial CAD solution. However, it's interoperability with other products is still lacking. On the other hand drawing can be exported in scalable vector graphics (SVG) format - allowing you to share finished drawings with a wide audience without requiring them to install the product.