Like I said back in January, SendToKindle acts something like a printer driver. You can use it to turn docs into PDFs and automatically email them to your Kindle account on Amazon.com. You can also right-click on a file while looking at it in a folder and send it to your Kindle account. This app sends the files to a specific email for a particular Kindle, not your general Kindle email address. But that's not a major issue because it will also archive the files in your Kindle Cloud Library so you can download them later. What's even better is that you can select DRM-free AZW and Mobi files and send them as is.
Note that it doesn't support any formats that Amazon wouldn't already convert for you. There are also better options for certain types of conversions. For example, while you could use SendtoKindle to convert webpages, dotEpub does a better job.