I use a number of tools here at RockingD Labs. As much as possible I use and support Open Source Software and Hardware. My present operating system of choice is Ubuntu 10.04 so you can be sure these tools work on that system.

This is a 2D drafting program by RibbonSoft. Their Open Source version is a few revisions behind their retail version, but it has been adequate for my needs. The price for the retail version is very economical.

QCAD reads and writes DXF files.

The Open Source version is available as a Ubuntu package, which makes it very easy to get started.