Tactile Switch Buttons (6mm slim) x 20 pack
Product Images and Video
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)