Want to do something retro with your distinctly modern Apple iPhone? You could change it into a vintage scientific calculator, courtesy of this HP app.
For the calculator obsessives out there, you can change the touchscreen of your iphone into a HP 12c, HP 12c platinum or HP 15c scientific calculator, each one a replica in terms of both layout and functionality, so perfect for boffins wanting some old school cool. Also if you turn it around, you get the same calculator with a difference layout (vertical to horizontal), although that also means you can't type rude words by flipping it upside down.
Depending on your choice of calculator, prices range from $14.99 to $29.99.