There is pretty much no free CAD software that can compete with AutoCAD. The problem starts with the DWG format is based on proprietary RealDWG that they license to vendors. Free software will not be given a license.
The problem migrates to AutoCAD being like C++ and having a long list of legacy objects that are never broken.
A competing format was headed by Bentley with OpenDWG but they abandoned it somewhat after RealDWG gave in to giving them the doormat for free.
Bentley has abandoned OpenDWG but the software that is built on it has continued to improve. IntelliCAD is not free, but there are enough companies building on it, that it would be difficult to kill.
My employer is a member of the Open Design Alliance [0] working on IntelliCAD. I work a layer up, building plugins for both AutoCAD and IntelliCAD. Free trials are not hard to find, so you can decide if it will work for you.
I am no expert in CAD software, my only experience is with FreeCAD - and I found it is pretty good for things I needed. Project evolves fast. There are PPAs with up-to-date binaries too.
The UI is really nice. Version 3.0 whenever it comes out is going to be awesome. You may also try FreeCad for things this can not model, but that has its own problems - particularly UI.
The OSS world still needs a full-featured NURBS library that supports trimming. Solvespace does, but the performance suffers as complexity increases.
