Free: LibreOffice Draw (meant to compete with MS Visio.)
Free: FreeCAD (2D/3D - Free But complicated interface.)
Free: Dassault DraftSight Free (Free but Requires Yearly Email Registration.)
[Looks like a AutoCAD Light clone.]
ADD (2017-12-12): I now recommend the LibreCAD fork of QCAD,
as the majority of developers from the old project migrated to the LibreCAD
project when the schism occurred. LibreCAD has much more development
activity and enjoys more regular updates than QCAD. LibreCAD is also free.
Cheap: QCAD (~37 USD) (Nice but I now recommend the LibreCAD fork.)
[There is a free edition, but it is very barebones. One layer only, etc.
But you can use it to see if you like the interface.]