Right now there's a beta test going on for a new, lightweight, free CAD program that works on Windows and Mac (and soon Linux). It support Autodesk's DWG format, and works very similarly to AutoCAD. It's a 2d environment only, so don't expect any 3d capabilities. It's called DraftSight, and you can get it here. It's made by Dassault Systems who also makes SolidWorks, Catia and several other professional products.
From their site:
DraftSight is a professional-grade, open 2D CAD product for users who want a better way to create, edit and view DWG files. DraftSight is easy to use and is available for professional CAD users, students and educators to download and activate for free.*
Based on an advanced architecture, DraftSight has a small footprint, should take a few minutes to download, and supports Microsoft® Windows XP®, Windows Vista®, Windows® 7 and the Mac®. (Linux® coming soon).
The download is around 60MB, and is obviously going after some market share currently owned by Autodesk. I gave it a try and found it to be very similar to Autocad - it even shares the same command line and keyboard driven interface if that's something you're already used to. The interface is simple (meaning it's nice), but definitely not as polished as the newly released AutoCAD for Mac, and it definitely doesn't have all of AutoCAD's features.