For 2D drawings
DraftIT is superb. There is a free version which works very well. There are fairly cheap (<£100) paid upgrades which add a lot of useful functionality, including DXF/DWG exports.
I've used this for my F1 project here:
http://www.oople.com/forums/showthread.php?t=42235
And my TC project (still in progress) here:
http://www.rctech.net/forum/electric...ect-tc6-c.html