So, you're trying to share some files with someone who only has access to an old version of Word. Or maybe that Microsoft-hating friend of yours who only uses WordPerfect. Perhaps you want that old-school ebook to be an epub, rather than .lit. For any of these problems, there are desktop apps that work, of course. However, sometimes you want to be able to do that within a program. If so, Doxument and the beautiful Doxument API are what you need.
From the Doxument website:
Doxument is designed for individuals, developers and businesses that have document conversion & OCR needs with their applications and require a flexible, reliable, cost-effective solution that can scale seamlessly. Doxument supports an array of document formats and can be highly customized by developers to meet a wide range of application needs.
Doxument is a paid service, but it is insanely cheap, paid per amount of data funneled through the API. It is RESTful, and return formats are JSON, YAML, XML and PHP. It could be used for all kinds of interesting things, such as making a service similar to Google Documents. It doesn't just do text documents either, also converting between various image formats. My favorite potential use would be integrating it into an eBook sharing site and using it for conversion of files between devices.
As it is a paid service, to use it you'll need to sign up and get an API key, and set up payment information. They currently have a PHP5 library available, and are looking for libraries for other formats. I suspect they'll show up soon, as this is a remarkably useful service available for pennies.