I would like to second recommendations for Google Sketchup, with this additional note for those who point out that some features are limited to the paid "pro" version.
If you are a student or faculty, you can obtain the pro version for free until the following July. They seem to be quite liberal in their interpretation of "student or faculty;" it apparently doesn't have to be an accredited university.
Our small farm cooperative teaches two-week intensive Permaculture Design Certificate courses, and submitting the syllabus was enough for them to grant us an educational license.
But you have to renew the ed license every July.
In addition to the lack of .DXF support, the other "biggie" is that you cannot create dynamic components. That should not be a big issue for most people, but it is fun to make doors that open when you click on them, etc. which you can only do in the pro version. But literally 95% of the functionality of Pro is in the free version, including the ability to use dynamic components created in the Pro version.