Electronics

Microcontrollers are nothing without any input devices (like sensors) or output devices (like displays, LEDs, servos, ...).

Simple Input/Output

Simple Analog Sensors

Pot3
Simple input device - 3 potentiometers
Accelerometer
Connecting a 2/3 axis accelerometer
TMP36
Temperature Sensor

Simple Output

LED
 
Relay
 
Servo Moters
 

Protocols

I2C

I2C
Overview
WiiChuck
Wii Controller Connector
DS1307 & DS3231
Real Time Clock (RTC)
24xx256
I2C EEPROM 256kbit
HMC6352
Digital Compass

SPI

SPI
Overview
74HC595
8-Bit Serial-In, Parallel-Out Shift Register
74HC165
8-Bit Parallel-In, Serial-Out Shift Register

UART

UART
Overview
FTDI
USB Serial Converter
GPS
LS20031 GPS-Bee
ELM327
on-board diagnostics interface
ID-12
RFID

1-Wire

1-Wire
Overview

MIDI

MIDI
Overview
MeeBlip
The Hackable Digital Synth
MCP4921
MidiVox Shield
VS1053
Musical Instrument Shield
ATSAM2195
Fluxamasynth Shield
YM2149
YM_MINI Synthesizer
Piezo
YM_MINI Synthesizer

Other Protocols

Ping
Ultrasonic Distance Sensor
Touch Screen
4-Wire Resistive Touch Technology

Microcontrollers

AVR

AVR
ATtiny & ATmega
ATmegaX8 (Arduino)
ATmega88 ATmega168 ATmega328
ATtinyX5
ATtiny25 ATtiny45 ATtiny85 (ATtiny13)

Arduino Shields (Daughterboards)

SpeakJet / TTS256
Voice and Sound Synthesizer

Useful Prototyping / Debugging / Reverse Engineering Tools

Poor Man's Oscilloscope
 
Poor Man's Logic Analyzer
 
Bus Pirate
 
Saleae Logic Analyzer
 

References

What connectors to use?
 
Resistor Values
 
Radio Control (R/C or RC)