progeCAD Professional is the most reliable 
Autodesk AutoCAD alternative, the low-cost 2D/3D DWG-native CAD, offering the AutoCAD look-and-feel, DWG 2014 and Windows 8 compatibility. It is a fully-functional, easy-to-use and low-cost CAD solution.
progeCAD includes exclusive highly productive features such as the PDF to DWG converter, the EasyArch plug-in with imperial measurements support, iCADLib integration with the TraceParts and Cadenas portals, and 3D modelling with ACIS solids. The progeCAD package contains both the standard tools of a generic CAD and the features typical to the most advanced CAD systems. Many users will immediately feel at ease with the menu, tools, icons, commands similar to AutoCAD, so any CAD professional accustomed to AutoCAD can easily get started with progeCAD.
A free 30-day trial version is available for download.
Moreover 
progeCAD Smart 2009 is a limited version available for free for private, non-commercial use only.
Danilo_Venom added progeCAD as alternative to HighDesign
Comments and Reviews
This is not a software. This is a joke. Unstable and unreliable, it is the worst cad that I've ever tried.
It is almost an Autocad. but if you are working with big drawings, xref's etc then is not the software you are looking for. If your job is based on the software, think twice and test it hard before you take the decision to buy it.
[Edited by gerrykoun, August 03]