1N4148 Diodes
$2.99
Just your standard silicon switching signal diode, works well when building your own keyboard. Great for allowing current to flow in one direction, but not the other.
Choose between a pack of 100 diodes or a pack of 500 for even more fun! Or maybe go full blast by getting a box of 5000.
SMD diodes are available as well, they are in a 0805 package.