There's QCAD, which is okay, but lacks some of the capabilities of software like AutoCAD. It's 2D only, but it's better than nothing.
http://www.qcad.org/qcad.html
There's also "TurboCAD", which I've never used, so I can give no review of.
http://www.cadandgraphics.com/tcmac3...FQ-QGgodH1Mpaw
Alternatively, you can use computer animation softwares, such as Lightwave, instead of actual CAD software to create 3D models of your robot. 116 has used Lightwave in the past for creating their plans for their drive systems and frames.