I use a variety of software on linux and Mac OS X that is open source for toolpath generation. My favorite at the moment is called HeekCAD/HeeksCNC. It's really alpha at the moment and can be a pain to install. So, that being said, if you're into circuit board milling, you might look at gcam:
gcam.js.cx/index.php/Main_Page
For just plain old 2D drawing, I like Qcad a lot now. It seems to come stock with most linux distros (or you just install it from Synaptic in Ubuntu).
I also run some windows applications under a program called 'Wine' which is a windows emulator. I have Autocad 2000, Rhino3d, Progecad, Vectorcam, and a few other programs working with it. Of course these aren't open source.
If you haven't already checked it out:
wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Cam
has a lot of good links to related cadcam software for linux.
I hope that helps a little.
Dan