Big Blue Saw Designer: a Free CAD Design Tool
This is the Big Blue Saw Designer Computer Aided Design software. You'll need to have Java installed to run this.
The Big Blue Saw Designer has support for constructive area geometry (CAG). AFAIK, this is unique among open source CAD programs, and rare among CAD programs in general.
Most CAD programs are designed as automatic drafting tools, not as tools to make real world objects. Computer Aided Drafting instead of Computer Aided Design, in other words. This program focuses on designing parts, rather than just making lines and circles.
You can combine or add shapes together simply by dragging them so that they overlap. To subtract, or make a hole, select a shape, right-click and choose "Subtractive" from the pop-up menu. The shape will then form a hole in the part.
The software is based upon jPicEdt.