I've played around with a few online services and the like, tried to turn this into that and that into this, trying to find a quick and easy service to turn various formats into Kindle's .Mobi format.
Well, I've finally found the easiest way, and it is awesome.
For one thing, it's free. Secondly, it's incredibly easy! Ready? Let's get started.
You will need to install only one piece of free open-source software. It's called Calibre, it's an ebook manager and reader, and it's going to do everything we need it to do, automatically. Works on Mac, Windows, even Linux.
Now that you've got that installed we need to add a book to it. You can do this in a couple ways: either hit 'Add Books' at the top left, or drag a book onto Calibre and it will auto-add it.
You'll see the book title pop up in the middle of the screen. Now plug in your e-reader, be it Kindle, Nook, or others, and Calibre will auto-detect your device. If you just want to read the book on your computer you can skip this step.
Select the book title by clicking on it once, and then at the top right hit 'Send to Device'. And you're done! Calibre will automatically reformat your book for the kind of file your device needs.
If you just want to read your ebook on the computer and want it in a specific format, you can use the 'Convert Books' button, top middle. In the window that pops up, check the right side and select the format you want, be that Mobi or Epub, etc. Hit okay, choose where to save it, and the reformat will begin.
Now, the first time I tried this I wondered why it seemed like nothing was happening. My book didn't show up where I saved it or anything. Then I saw the 'Jobs' counter in the lower right corner of Calibre's window. It can take about a minute to reformat a single ebook, so be patient. I wasn't that first time!
Why don't you now head down to Project Gutenberg, download some of those public domain classics you've always wanted to read and put them, in the space of a minute, on any device you like and in any format.