Released on November 19, 2007, the $359 Amazon Kindle is a stand-alone electronic book reader that utilizes electronic-paper / e-ink technology and is capable of holding over 200 titles on its internal memory. The Kindle's primary competitor is the Sony Portable Reader, but Amazon differentiates the Kindle from other ebook readers via Amazon's wireless Whispernet connection over Sprint's EVDO network. This connection is provided for free by Amazon with no monthly cost. The Amazon Whispernet connection allows the Kindle to purchase books directly from Amazon's Kindle Store from wherever a cellular connection is available, access Wikipedia.org, and browse the web via a basic custom web browser. Besides reading Amazon's proprietary .azw ebook file format, the Kindle can read text files (.txt), Mobi ebook files (.mobi, .prc), and it can play both MP3 files and Audible (.aa) audio book files that can be transferred to the Kindle via a USB connection or with an optional SD-card. Users can have personal files such as Microsoft Word documents, HTML, or pictures (JPEG, GIF, PNG, or BMP) converted to Kindle format for a small fee by emailing these documents to the device.