Raspberry Pi development hat
Raspberry Pi motor hat

Raspberry Pi single board computers can have more features added using an extra circuit board fitted to the top.
This can add sensors, buttons, displays, higher voltage controls e.g for motors, data storage, audio devices, relays etc.
See a list of Raspberry Pi computers that suite Internet of Things

Raspberry Pi hats popularity by descending sales from www.aliexpress.com

PopularityDescriptionCost US $
1DIY Proto HAT Shield. Similar to the top left image with Veroboard like copper tracks present. 1.98
2Raspberry Pi 3 Power Adapter 2500mAh Li-ion Battery Power Supply18.89
3Raspberry Pi 3 Motor HAT Full function Robot Expansion Board 20.89
4Raspberry PI Power Pack UPS HAT Lithium Battery Expansion Board Power Supply7.69
5Raspberry Pi Sense HAT with Orientation Pressure Humidity and Temperature 38.99
6Programmable Smart Temperature Control Fan+Power Hat Board13.29
72.2"TFT Screen LCD Display HAT w/Buttons IR Sensor12.29
82.13inch e-Paper HAT 250x122 2.13inch E-Ink Display20.89
9Raspberry Pi Prototyping Hat/Shield/Expansion Board with Screw Terminals12.20
10True color RGB LED HAT colorful display13.99
11Display Development Tools Module RGB Matrix HAT + RTC (real time clock)38.00
12Raspberry Pi 2B Expansion Board Digital input / output / 2 relay / 4 switch / 8 LED DIDO HAT45.58
1316-Channel PWM / Servo HAT (control motors)26.67
14Capacitive Touch HAT23.33
15Pimoroni Explorer HAT with breadboard and development board35.00
16Ultimate GPS HAT (add longitude and latitude values to your project)67.00