I have recently been looking at different CAD options on Ubuntu and LibreCAD (http://librecad.org/) is looking like the best option for my needs at current (apart from the wish there would be some DWG support).
Since the main Ubuntu repositories are usually a fair bit behind the lastest stable realeases of LibreCAD you need to add the LibreCAD Dev PPA Repository
sudo add-apt-repository ppa:librecad-dev/librecad-stable
sudo apt-get update
sudo apt-get install librecad