From Wikipedia:

Amazon Kindle is an electronic book (e-book) device launched in the United States by Amazon.com in November 2007. It uses an electronic paper display, reads the proprietary Kindle (AZW) format, and downloads content over Amazon Whispernet, which uses the Sprint EVDO network. This means that the Kindle can be used as a stand alone without a computer. Whispernet is accessible through Kindle without any fee [1]. On the release day, the Kindle Store had more than 88,000 digital titles available for download [2]. Amazon's first offering of the Kindle sold out in five and a half hours [3]. It retails for $399 from Amazon.com.
More info and videos on Amazon.com

Technical specifications, from Wikipedia:

The Kindle features a 6" diagonal, 4-level grayscale electrophoretic display (E Ink material) with a resolution of 600×800 pixels (167 ppi), although the largest graphic image that can be displayed without being resized is 450x550 pixels. It measures 5.3 inches × 7.5 inches × 0.7 inches (134.5 mm × 190 mm × 19 mm) and weighs 10.3 oz (295 g). The Kindle's internal storage capacity is 256 MB, shipping with 180 MB free. A SD memory card expansion slot is present, officially supporting up to 4GB which implies support for SDHC. It has 64 MB of RAM. The battery lasts roughly two days with wireless on, and one week with wireless off. The battery charges in about two hours. A USB 2.0 port (mini-B connector) is available for connecting to a computer (where it acts as a USB flash drive). The Kindle features a headphone jack and one-year warranty. The device runs on a modified version of Linux based on the 2.6.10 kernel.
The Kindle uses Linux

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Sony Reader and Amazon Kindle, compared