When Hoplite Industries needed some overpriced spacers, I was able to 3D print some suitable alternatives for a fraction of the cost using OpenJSCAD! Watch my talk at Big Sky Dev Con in which I talk about making real things using code.
I've read that dowel joints are nearly if not as strong as mortise and tenon joints. The mortise and tenon joint requires expensive tools (specifically a table saw and a router) and the know how to make them, but dowel joints can be made with just a drill. The tricky part, though, is getting your dowel holes lined up on each piece. With my dowel jig widget, you can configure a jig for your specific situation and 3D print it at home or for an affordable price online.
I designed and 3D printed a gear shifter for my car. An engineer experienced with direct metal laser sintering wanted to to make his own out of titanium. In exchange for the model, he made me one as well. Read about how I barely managed to tap threads into it and see how you can do it yourself in a stainless steel version.
O'Reilly Media's Oriole platform provides a rich learning experience by combining video, text and interactive coding widgets. To me, it seemed like the perfect way to teach Arduino, but interfacing with Arduino from a browser is tricky business. Read about how I managed to get a simple example going that includes writing code, uploading it to your Arduino and using your Arduino to interact with widgets on the page, all from your browser window.