Adafruit Tactile Switch Buttons (6mm tall) x 10 pack
Super-tall 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. These are extra tall which are useful when putting your project in an enclosure and you want the buttons to stick out.
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