Arduino is an open-source electronics prototyping platform based on plug-in hardware and user-configurable software. Arduino is in wide use by robotics hobbyists.
The Raspberry Pi is a credit-card sized mini-computer (85.60mm x 56mm x 21mm) based on the ARM1176JZFS microprocessor that can plug into your TV via an HDMI interface cable, and connect to your USB mouse and keyboard. Sales of the Pi commenced in February 2012. Its popularity and reception have been enormous.
The latest Model B version has a maximum of 512MB RAM as well as 2 USB ports and an Ethernet port. There are 8 dedicated GPIO pins, a UART, an i2c bus, an SPI bus with two chip selects, i2s audio, and power rails of 3v3, 5v, and ground.
If you don't want to use the extra low level interfaces (UART, i2c, etc) then the Revision 2 Pi can support 21 GPIOs, and the maximum number of GPIOs can theoretically be indefinitely expanded by making use of the i2c or SPI bus.
The whole operating system fits on a plug-in SD card, so you can change operating systems simply by inserting a different memory card. The Pi comes with NOOBs (New Out of Box Software) that lets you select your operating system for installation - the default being Raspbian Wheezy, a linux variant. With this install, the Pi comes with the python programming language.
Amongst other things, the Pi can be used for electronics projects, spreadsheets, word-processing, robotics, games, and high-definition video. AusPi Technologies sells Pis to the Australian market.
Makey Makey can be used as a game controller for student MIT Scratch projects, or as input to the Raspberry Pi to jack-up the learning and the fun. AusPi Technologies sells Makey Makey boards to the Australian market.
Here are some example MIT Scratch projects for Makey Makey.
Beaglebone is a credit-card-sized Linux computer that connects to the Internet and runs the Android and Ubuntu operating systems.