Adafruit Tactile Switch Buttons (12mm square, 6mm tall) x 10 pack [ADA1119]
Medium-sized clicky momentary switches are standard input "buttons" on electronic projects. These work best in a PCB but can be used on a solderless breadboard as shown in this tutorial. The pins are normally open (disconnected) and when the button is pressed they are momentarily closed.
They come in a pack of 10, they're pretty handy so you'll want to keep them around.
You can find an EAGLE cad package for these in switch-omron:B3F-40XX