Commonly used sensors

Here are suggestions for the most commonly used sensors by beginners. There are of course many variations possible for each of these, this is just a sampling. To purchase, check with local vendors first. The links below are to online vendors, note these change frequently, so a newer or cheaper option may be available. 

Useful video overviews of sensor basics:
https://itp.nyu.edu/physcomp/videos/videos-sensors/

light (basic): photocell (https://www.sparkfun.com/products/9088)

light (precise): Ambi (http://moderndevice.com/product/ambi-light-sensor/)

distance (proximity): Sharp IR sensors (short range https://www.sparkfun.com/products/8959, medium https://www.sparkfun.com/products/242, long https://www.sparkfun.com/products/8958), Maxbotix ultrasonic sensors (https://www.sparkfun.com/products/8502 etc.)

proximity switch: magnetic reed switch (https://www.sparkfun.com/products/10601), hall-effect magnetic sensors (http://bildr.org/2011/04/various-hall-effect-sensors/)

touch, slide/ribbon: linear softpot (https://www.sparkfun.com/search/products?term=softpot), rotary softpot (http://www.adafruit.com/products/1069)

touch, non-contact (cheap): DIY capacitive http://playground.arduino.cc//Main/CapacitiveSensor

touch, non-contact (better control): capacitive breakout board https://www.sparkfun.com/products/9695

flex/bend: flex sensor (short https://www.sparkfun.com/products/10264, long https://www.sparkfun.com/products/8606)

stretch/pressure – fabric: EeonTex Conductive Stretchable fabric and Pressure Sensing fabric

rotation: potentiometer (10k linear https://www.sparkfun.com/products/9939, knob https://www.sparkfun.com/products/10001)

pressure: force sensitive resistor (round https://www.sparkfun.com/products/9375, square https://www.sparkfun.com/products/9376)

sound: Sound Detector – volume (http://tinkersphere.com/sensors/919-microphone-sound-db-level-detector-arduino-compatible.html), volume or threshold (https://www.sparkfun.com/products/12642)

sound (MIDI): MIDI Shield – https://www.sparkfun.com/products/12898

tilt (basic): tilt switch (https://www.sparkfun.com/products/10289)

tilt/rotation (advanced): accelerometer (https://www.sparkfun.com/pages/accel_gyro_guide or http://www.adafruit.com/search?q=accelerometer or http://moderndevice.com/product/3-axis-accelerometer-module/)

vibration: piezo (https://www.sparkfun.com/products/9196)

temperature (basic): TMP36 (https://www.sparkfun.com/products/10988)

temperature (precise): TMP102 (https://www.sparkfun.com/products/11931)

wind: anemometer (http://moderndevice.com/product/wind-sensor/)

heartbeat: Pulse Sensor (https://www.sparkfun.com/products/11574 or http://moderndevice.com/product/pulse-heartbeat-sensor-pulse-sensor-1x/)

location (GPS, outdoor): Adafruit GPS products, Sparkfun GPS buying guide

location (indoor): Location, Pozyx, RadarCat

LOTS MORE:

https://www.sparkfun.com/categories/23?page=all

http://www.adafruit.com/category/35

http://playground.arduino.cc/Main/InterfacingWithHardware#InputTOC

Note that many of the above sensors are more complex and require more work to interface with. Note also some of the units from Sparkfun are sold alone or with breakout boards (additional circuitry added) – if given the option, choose the breakout board, that will save you hours making your own.