Many people who purchase the Amazon Kindle will simply use it to purchase books, magazines, and blogs from the Amazon Kindle store and go from there. Are books from Amazon.com are the only ones that are compatible with the Kindle?

The Amazon Kindle has an available SD memory card slot and supports up to 4GB SD memory cards.

Users can download content from Amazon in the proprietary Kindle format (AZW), or load unprotected Mobipocket (PRC, MOBI) or plain text content.

There are MANY source for free ebooks. Here are a few:

• ManyBooks.net
• Project Gutenburg
• Baen free Library
• Mobipocket free ebooks
• Fictionwise free books

ManyBooks.net has about 20,000 free ebooks, and they have already added "Kindle" to their drop down menu (download format selection). I have been using these sites for years for reading material and have an SD loaded up with content for my Kindle.

From the Amazon Kindle page:

Eliminating the need to print, Kindle makes it easy to take your personal documents with you. Each Kindle has a unique and customizable e-mail address. You can set your unique email address on your Manage Your Kindle page. This allows you and your contacts to e-mail Word documents and pictures wirelessly to your Kindle for only $.10. Kindle supports wireless delivery of unprotected Microsoft Word, HTML, TXT, JPEG, GIF, PNG, BMP, PRC and MOBI files.

 
PDF conversion is experimental. The experimental category represents the features we are working on to enhance the Kindle experience even further. You can email your PDFs wirelessly to your Kindle. Due to PDF's fixed layout format, some complex PDF files might not format correctly on your Kindle.
If you are not in a wireless area or would like to avoid the $.10 fee, you can send attachments to "name"@free.kindle.com to be converted and e-mailed to your computer at the e-mail address associated with your Amazon.com account login. You can then transfer the document to your Kindle using your USB connection. For example, if your Kindle email address is Jay@Kindle.com, send your attachments to Jay@free.kindle.com.

If you don't want to deal with emailing files you can download Mobipocket eBook creator (free, make sure and get the publisher version) and I think you need to have Mobipocket dektop reader (free) installed on you computer.  With Mobipocket creator, you can convert word, text and pdf files for your Kindle without much effort. (If I can do it, you can too!)

The Kindle also supports audio in the form of MP3s and Audible 2, 3, and 4 audiobooks, which must be transferred to the Kindle over USB or on an SD card.

Amazon books only for the Kindle? The answer is NO! I have over 100 ebooks, (including converted word documents and PDF files) on my SD card and the Kindle reads them all.......

View Original Blog Post