Combine your smart phone or tablet with a 3D-printed clip and glass sphere to create an inexpensive, yet powerful, microscope. The microscope slips over the camera lens of the cell phone and is no thicker than a phone case. It's designed to fit most cell phone brands, including iPhone 4 and 5, Galaxy S3 and S4 phones and iPads. The material cost, not including the printer, is under $1.
For full details go here: http://availabletechnologies.pnnl.gov/technology.asp?id=393
For other examples of open source scientific hardware you can print see Open-source Lab