There are few things I like more than seeing someone come up with a startlingly low-cost solution to an otherwise expensive problem. Instructables user Yoshinok has done just that by turning a few pieces of Plexiglas, a laser pointer and an iPhone into a highly capable digital microscope. The entire project cost around US$10 (not including the iPhone itself, of course), but the results are downright amazing.
Using the focus lens from a cheap laser pointer as the magnification device, Plexiglas slides to support the phone and items being inspected, and a wooden base, the setup is capable of 175x magnification, which is apparently enough to spot individual plant cells. Wow.
Yoshinok appears to be using an iPhone 4 or 4s in the instructional video, but any iPhone (or even iPad) would likely work just as well. He notes that the microscope stands could be used in schools that don't have budgets for something as expensive as a digital microscope -- and given the number of students walking around with smartphones these days, it makes a lot of sense.