TIP120 - Transistor

This beefy transistor is perfect for controlling a DC motor with a PWM signal from your Arduino. I like these transistors because they are overkill for most projects – meaning there is less of a chance I will burn it out while testing out a circuit.
The official Arduino page has a good write up for this transistor and example code.
SSD1306 - OLED Module

This display is cool. I like the ones that have yellow on top and blue on the bottom because it differentiates the top like like a header.
These require the Adafruit_GFX library and documentation can be found here at Adafruit.
The board has 4 pins:
- GND – The Ground connection.
- VCC – The power supply, 5 VDC.
- SCL – The I2C Clock.
- SDA – The I2C Data line.
L298N - Dule H Bridge Stepper Motor Driver

Making stuff move is one of the coolest things you can do with an arduino. This classic chip is perfect for driving a stepper motor.
As usual The DroneBot Workshop has a good write up and example code for this chip.
KY-040 Rotary Encoder Module

As usual The DroneBot Workshop has a fantastic write up about how to use a Rotary Encoder.
DS3231 AT24C32

Real Time Clock
DHT22 / AM2302

A more precise temperature and humidity sensor than the DHT11. A good write up about the differences is at Seed Studio.