Tactile Switch Buttons (6mm slim) x 20 pack
Slim clicky momentary switches are standard input "buttons" on electronic projects. These are half the width of classic 6mm tactile switches so they line up better on a breadboard, just plug them into every-other row. These work best in a PCB but can be used on a solderless breadboard as shown in this tutorial. The two pins are normally open (disconnected) and when the button is pressed they are momentarily closed.
They come in a pack of 20, they're pretty handy so you'll want to keep them around.
Tactile Switch Buttons (6mm slim) x 20 pack (2:10)