Cailbre is far and away the best ebook conversion and library management software now available. What makes it more amazing is it’s free. Think of it as iTunes for your books–it will let you import files, convert them to other formats (txt to epub, for one example), and sync them up to your device. If you use the Stanza app on your iPhone or iPod Touch, you can even turn Calibre into a local server and access your files from your mobile device. I like Calibre so much that I donated money to the developer!

Stanza will convert all kinds of text into several ebook formats, including the proprietary .azw format Amazon uses for its own Kindle texts. You can also use Stanza to read texts directly on the screen in a clutter-free layout. Apple and Windows.

Mobipocket Creator will produce .prc files that can be displayed on your Kindle. Windows only.

You can also use Mobipocket Desktop to manage your ebooks and RSS feeks, then sync them to your Kindle whenever you connect it via USB. However, you have to use version 6.0, not any of the later versions (such as 6.1 or 6.2).  Amazon, which owns Mobipocket, has removed support for the Kindle from more recent versions. (Shame on you, Amazon!)

Update: Here’s a post I wrote that explains how to convert screenplays and scripts for reading on your Kindle.