1- PDF 4 Kindle
Converts .pdf toKindle format .mobi or .azw Plus, it's free! Online eBook converter, no download required. Supports MacOS, Windows and Linux.
2- Send to Kindle
Send to Kindle is a cool application that allows users to send documents from their computers to their Kindles or other devices such as iPad and iPod provided they have Kindle app installed on them. This is a great service by Amazon and is completey free. It works on Windows XP or Windows Vista or Windows 7.
3- Many Books
This is a site that provides a daily selection of around five to ten new books to read. All of the books are free and can be downloaded in a variety of formats: HTML, AZW, MOBI, and RTF, to name a few.
4- Mobi Pocket Creator
This is an application to create your own eBooks based on blank pages or pre-designed templates. But better yet, you can convert your PDFs, currently unreadable on the Kindle and Kindle 2, to friendly PRC file formats. Amazon charges ten cents to make the conversion itself
Kindlefeeder is a service for Amazon Kindle owners that lets you aggregate your favorite feeds and have them delivered to your Kindle in a convenient, easy-to-navigate eBook.