$25 Raspberry Pi Computer available January 2012

Raspberry Pi the credit card sized Linux based Computers will be available in January 2012

Model B - $35 - 256 MB Ram with 10/100 wired Ethernet
Model A - $25 - 128 MB Ram, No Ethernet 
(WiFi via USB  dongle)

The exact release date has not yet been confirmed, however, you can sign up for a notification email.
Join the Raspberry Pi Mailing List.

The 45g computer has a Linux based operating system which receives power via a 5volt micro USB but can also be powered with 4 x AA batteries.  Features include:

• RCA Video
• 3.5mm Audio Jack
• USB 2 slot 
• HDMI 
• SD Card slot
• Ethernet (model B only)
• 85.60mm x 53.98mm x 17mm approx

The computer like the Mac Mini is soley a computer with no external devices.
To use connect the Raspberry Pi to an analog, digital TVor DVI monitor then via a USB hub add a keyboard, mouse, external storage, network device, WiFi Dongle, speakers etc.


Source and More Information:  Faqs about Raspberrry Pi

UPDATE - 3 June 2013 & 4 June 2013 :
Need help setting up your Raspberry Pi - New Out Of Box Software (NOOBS) makes it a whole lot easier.