A lot of people are getting very excited about the imminent launch of the Raspberry Pi. It's an ultra-low cost Linux-capable board based on an ARM11 core, and it's due to go on sale in December.
If you want a quick overview of the Pi and what it can do, this video on the foundation website is a good place to start.
The Raspberry Pi foundation expects strong early demand from hobbyists, but the longer-term aim is to attract new users at school level. Since the Pi is to sell for about £20-£25 that looks realistic.
Youcan't pre-order the Pi, and should beware of sites offering the Pi: they are trying to part you from your money. You can help to test out the new on-line shop and contribute to this very worthwhile project by purchasing a Raspberry Pi sticker. You'll end up paying a little over £3, most of which will go to support the foundation.
If you want to use an ARM-based Linux board for robotics or test gear applications you are spoilt for choice at the moment. There's the Beagleboard, the mbed, the LPCxpresso, and the gumstix range. Soon there will be the BeagleBone and Rapsberry Pi. While their markets overlap a little, each board has its own unique advantages. Once I've got my hands on the BeagleBone and Pi I will attempt a feature-based comparison and post the results.
While you're waiting, do register at the Raspberry Pi site and buy a sticker!