There are lots os free alternatives to CAD softwares these days like FreeCAD and DraftSight that will work really well for people trying to go avoid really expansive tools like AutoCAD. This week I found another interesting project that might become an alternative to CAD, and it is all web based. The Web-CAD project uses HTML 5, CSS and lots of javascript to create something really close to a CAD software, but running on a web browser. This is not the first web based CAD tool on the market, but as far as I know is the first one that allow us to download the source code, after all is all based on HTML.
If you want to give it a try, you can download it from the project page.
I think JS-CAD is more promising, and is free to download.
http://sourceforge.net/projects/js-cad/
Online demo:
http://js-cad.sourceforge.net/latest/index.html
There’s more to find if you’re looking for free cad alternatives:
Doublecad (windows, not open source)
Draftsight (all platforms, not open source)
Librecad (all platforms, open source)
Nanocad (windows, not open source)
Cademia (community edition, all platforms)
These are all pretty good developed and maintained, maybe you can comment wich one(s) are best for using in combination with blender?