Draftsight, an AutoCAD clone, is a good option if you are just interested in 2D work. OnShape was mentioned above. It will get you into 3D solid modeling.
If you don't mind paying for CAD software Visual CAD is a good 2D package. GeoMagic is a good 3d Solid modeling package. I use both of these packages, depending on the job. GeoMagic will make 2D drawings, but sometimes it is just quicke to do the work in 2D. OnShape does look interesting to me.