In a small change from the norm, here are a plethora of DIY guides for building speaker systems & boxes. So if you're an audiophile be sure to check out this page, you won't be disappointed!
The midi drum project is somewhat of an antique being that it began in '91. It uses 8051's to create realistic drum beats depending on which drums you hit. Much more complicated than it sounds. The many circuit diagrams and schematics are included along with the source code.
While building your own set of noise canceling headphones might turn out a bit big and bulky the theory behind how they work makes this project worth a read. Noise canceling headphones hit commercial markets a few years ago and are now very popular, especially among business travelers.