A new, interesting board aimed at helping kids get into programming is the BBC micro:bit.
They have setup a Foundation to support this device, and they have a lot of information on their website.
You can purchase them from several sources. In the US, two sources are Adafruit and Sparkfun. (see the website for a list of re-sellers worldwide) Both sell the board at about $15, tho you can get a "kit" that includes a USB cable and a battery pack for a couple more bucks. Both sell an edge connector for the cards, but Sparkfun has one that allows for the board to be attacked to a breadboard HERE.
The front of the board has 2 buttons and a 5x5 set of LEDs.
On the back you can see the "guts" of the board. There is a micro-USB port, a reset button, and a battery pack connector.
You can see the microcontroller, which has a 32bit ARM Cortex CPU with 16K RAM. (so this board is more like an Arduino then a Raspberry Pi), and also has Bluetooth Low Energy (you can download to the board over Bluetooth rather then USB). This is a Nordic nRF51822 microcontroller. Another microcontroller is onboard, a NXP/Freescale KL26Z which helps out. In addition there is a compass/metal detector and an accelerometer.
Along the bottom edge of are connectors for power, ground and 3 analog/digital input, as well as other pinouts.
Most of the information on how to use these boards is at the website. There is full info on how to program them (several platforms to use, the preferred is MicroPython), as well as how to use them in hardware projects.
As these are pretty new, I expect we'll see more info on them. These can't (and shouldn't) replace Arduino or Raspberry Pi, but should be a new tool for educating everyone in programming that can work along side those.
The BBC aims to get these in the hands of kids all around the UK. And there is interest in this board in other countries. You might wonder why the BBC is doing this, but they have a history of promoting programming among school kids. Back on the 80s they had a tv show and tying to it there was the BBC Micro, made by Acorn Computer Company. A 6502-based system, the BBC Micro competed against other 8-bit systems from Apple, Atari, and Commodore. Acorn created the Acorn Risk Machine architecture which continues as ARM, used in pretty much all smartphones and tablets. as well as most microcontrollers and SOCs used in single-board computers. (AVR and Intel-based make up the rest).
The success of the BBC Micro is what inspired the people behind the Raspberry Pi Project. So now the BBC is getting back into the field they pretty much inspired. Will be interesting where this goes.