Adafruit Magnetic contact switch (door sensor)
Adafruit Product Code: 375
This sensor is essentially a reed switch, encased in an ABS plastic shell. Normally the reed is 'open' (no connection between the two wires). The other half is a magnet. When the magnet is less than 13mm (0.5") away, the reed switch closes. They're often used to detect when a door or drawer is open, which is why they have mounting tabs and screws. You can also pick up some double-sided foam tape from a hardware store to mount these, that works well without needing screws.