If you want to design technical stuff and are ok with programming then OpenSCAD is a good choice.
http://www.openscad.org/
There is also implicitcad but it lacks a good quick preview yet.
http://www.implicitcad.org/

I've written a minimal language for the sage CAS system: miniSageCAD
It's quite slow but can generate cool stuff:
https://www.thingiverse.com/thing:40210