LibreCAD is a free 2D CAD drawing tool, and its old name is CADuntu. It is running on Windows, Mac OS, and Linux, and supports read/edit/create CAD files with .dxf extension.

Ubuntu user can get LibreCAD by adding following PPAs.

UPDATE: For the stable versions, hit Ctrl+Alt+T to launch terminal, run following 3 commands to install LibreCAD (support 12.10, 12.04, 11.10, 11.10, 10.10):

sudo add-apt-repository ppa:librecad-dev/librecad-stable
sudo apt-get update
sudo apt-get install librecad

For the latest develop version, install it from daily build ppa. Hit Ctrl+Alt+T to launch terminal, then run 3 commands:

sudo add-apt-repository ppa:librecad-dev/librecad-daily
sudo apt-get update
sudo apt-get install librecad

Open up a terminal window from Applications -> Accessories menu, and Ubuntu 10.04 type following commands:

sudo add-apt-repository ppa:showard314/librecad
sudo apt-get update
sudo apt-get install librecad

Ubuntu 10.10 and 11.04 use this:

sudo add-apt-repository ppa:showard314/ppa
sudo apt-get update
sudo apt-get install librecad

After installation, start it at Applications -> Graphics -> libreCAD.

Homepage of LibreCAD: librecad.org