free if you use dxf format but if you whant the ability to save and open native Autodesk dwg you need to buy. below the link for windows 64bit.
QCAD Community Edition
If you are looking for the free open source QCAD Community Edition, you can download the trial version for your platform (see above) and then remove the QCAD Professional add-on running in trial mode (click Remove in the Trial widget and follow on screen instructions).