Lacking a fancy name and a fancy interface, and still in beta, Auto Kindle eBook Converter nevertheless quickly converts some of the most-used desktop formats to a Kindle-friendly MOBI for free.
Compatible with PDF, HTML, LIT, PDB, and CHM files, the program opens to a Windows file browser. Choose the file you want to convert, hit Open, and then choose your destination folder. The converter goes to work, generally converting files quickly--although this depends on the length of your document. When it's done, connect your Kindle to your computer and move the file into the Documents folder. Safely disconnect the Kindle, and the converted file will appear in your main list of documents, automatically converted on the Kindle to its default MPB format.
The freeware converter is dead simple but there's plenty of room for improvement. The most obvious thing it's missing is a graphical interface, which could be combined with a default output directory to cut out several steps. Some formats may also hiccup on images, so don't be surprised if pictures don't always convert. However, it works and offers users another option besides e-mailing documents to their Kindle and paying the small conversion fee.