Raspberry Pi

I love RPi for 24*7 headless usage. Hope they produce Intel Atom versions too. There are others that do so but you know.... RPi has the greatest community & support.

Easy to use and small with more power than it looks like.

I use mine to have a server running day and night. if you have problems with heat just buy a radiator and a fan and the server.
The system only support ARMv6 systems and not AMD64 as normal systems do so it can't have all types of operative systems on it. It is just as AMD64 system's can't have ARMv6 operative systems on it. But use Raspian if you just want to learn programming with it.

A list of reasons why you would want to use a raspberry pi:

• You need a portable computer to embed into hardware (smart mirror)
• You need a low-powered computer that needs to run all day and night (Plex server / torrenting)
• You need a PC to act as a server dedicated to one specific task (Running a VPN, twitter bot, etc)
• You need to integrate with an arduino to have a device that can run all day and handle inputs / outputs
• you need a local server / PC

[Edited by Kagerjay, July 07]