There are a multitude of sensors for various physical efffects. Check eBay for more. Most are between $1.00 and $5.00 and ship from china in about 2-3 weeks.
Passive Infra Red motion Sensors (PIR)
Touch Sensor Capacitive Switch Module TTP223B
Infrared Reflective Proximity Sensor Modules
![water-level-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/water-level-sensor-200px.jpg)
![ky-023-joystick-axis-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/ky-023-joystick-axis-sensor-200px.jpg)
![hc-sr04-ultrasonic-distance-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/hc-sr04-ultrasonic-distance-sensor.jpg)
![dht11-temp-humidity-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/dht11-temp-humidity-sensor-200px.jpg)
![mq9-gas-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/mq9-gas-sensor-200px.jpg)
![mq-3-ethanol-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/mq-3-ethenol-sensor20opx.jpg)
![mq2-gas-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/mq2-gs-sensor-200px.jpg)
![3-axis-sensor](http://ham.aditl.com/wp-content/uploads/2015/01/3-axis-sensor-200px.jpg)
![ky-008-laser-transmitter](http://ham.aditl.com/wp-content/uploads/2015/01/ky-008-laser-transmitter-200px.jpg)