My son won a Kindle Touch a few months ago, so now whenever I see a new Kindle utility or service I am quick to notice.
While the Windows version of Send To Kindle was released a few months ago, the Mac version was just recently released. This app allows you to convert nearly any document to Kindle format and then send it to your Kindle (or Kindle app on your smartphone).
You can use drag and drop to convert a document or even the "Print to Kindle" command that gets added to your Mac when the software is installed.
I think i will be used for converting longer documents and articles I find on the web so that I can read them when I am out and about or simply don't want to use my laptop or iPhone for reading. If you have a Kindle, this app might help you convert some more reading material to use on it.