Here is a new, free service called RetroRead that is sponsored by BLTC Press. I think is is a great idea, as Calibre is not very easy for a novice to use and there is a real need for a simple, free service for those who are not computer gurus. I will even use it myself, as it is certainly much easier to stay in my browser, rather than to go through the whole download and conversion process with Calibre.
RetroRead is a FREE publishing service that allows you to automatically convert any of the hundreds of thousands of free public domain books available in EPUB format at books.google.com, and have them sent either directly to your Kindle, or emailed to your regular email address, in the native MOBI format that your Kindle understands.
RetroRead is also a community-created library of all of the books others have been converted, and which you can download for free. Find converted titles by searching the RetroRead library using Google Book Search. Registration is not required.