Partial list of devices we can use:
Pushbutton
Reed relay
Photo interrupter
Rotary encoder
Digital Hall sensor
Passive infrared motion detector
Infrared proximity sensor
Photoresistor
Thermistor
Linear Hall sensor
Ultrasonic range sensor
Matrix keyboard 3x4 and 4x4
Mini joystick module
MOSTFET module
relay module
servos
ULN2003 motor driver module (see
stepper
).
ENC28J60
ethernet module
W5100
based Arduino ethernet shield
TM16xx interface
MAX7219 based led matrix (using for example LedControl library)
1602 character HD44780 LCD module via LiquidCrystal (or via LiquidCrystal_I2C) library
Nokia 5110 (PCD8544) LCD module.
PCD8544 datasheet
.
SD cards
via adapters and SD library
432 MHz transmitter/receiver via for example rc_switch library
74HC595
shift register