Polish engineering student mb1988mb1988 and two friends built this homebrew pulse oximeter. It measures oxygen saturation in a patient by shining two wavelengths of light (infrared and red) through their finger, and recording the changes in light transmission over time. To power it all, they are using an 68HC908AB32 processor.
Building a device like this is complicated enough, but they went the extra mile to build an excellent-looking prototype, complete with a graphical GUI and case styling. The circuit schematic is available on their forum post. [via hacked gadgets]