Inkscape
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format.
Inkscape is an excellent tool to work with architectural
visualisations. In the Seminaarinmäki project we have used Inkscape for drawing floor plans. You can
find tutorial of that process from drawing floor plan link.
Downloads
Many Linux distros has Inkscape allready installed. For Mac and Windows it can be downloaded from: http://inkscape.org/download.php