Web-CAD free alternative to AutoCAD based on HTML 5
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.
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?