I am sure you heard the news about the new Raspberry Pi model with the code name Zero. It still amazes be how such advanced technology can be packed in a tiny pcb and purchased for only 5£.
The specs:

• 1Ghz, Single-core CPU
• 512MB RAM
• Mini HDMI and USB On-The-Go ports
• Micro USB power
• HAT-compatible 40-pin header
  
• Composite video and reset headers

You can buy it from element14, The Pi Hut, Pimoroni or Adafruit Micro Center.