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.
Changes to defaults for ellipses and splines. Added "Make Rectangular". Added geometry information to parallelogram editor. See the blog post.
2008-09-02
Maximum part size increased.
2008-07-24
Macintosh bug fixed.
2008-06-30
Your currently active file is saved in your My Files section if you are logged in.
2008-06-25
Improved dragging of drawing elements. You can now easily drag several selected drawing elements at once.
Improved snap-to-grid allows you to snap the moved item back to the original position.
Improved keyboard shortcuts:
Ctrl-X: Cut
Ctrl-C: Copy
Ctrl-V: Paste
Ctrl-Z: Undo
Ctrl-Y: Redo
2007-08-20
Applet is now resizable within the window.
2007-08-19
Made applet appear embedded inside the web page. Added save feature so that you can return to the designer page in the same session and keep your existing design.
Added Java version checker applet. Made default drawing size bigger.
2007-07-04
Numerous user interface changes, including adding online help, making the Esc key do something useful, and adding buttons for togglng the subtractivity of a drawing object and editing an object's geometry.
Much better interaction with the website, including working around popup blockers
