Although Calibre is a great tool for managing and converting ebooks, it runs on the computer that it's installed on.
In my case we have a few different computers in the house, and so being able to grab a (DRM-free) document off the web and converting it from any computer and then dropping it on the kindle is useful.
I happen to have a media server PC that runs 24/7 and so I decided to create such a web service for use by others on my home network.
And, I thought I would share some of the things I found on the web that helped me put this service together.
xampplite is a nice, easy to install apache/PHP (and other things) web server.
This can be used to create some simple web pages and PHP scripts that run, for example, Calibre's ebook-convert.exe tool to convert a downloaded epub document to mobi format for use on the Kindle. Basically, you throw .html and/or .php files under the htdocs directory where xampplite is installed and you're ready to go.
This page.
http://www.tizag.com/phpT/fileupload.php, provides a nice tutorial on how to write an HTML form and accompanying PHP to upload a file to the server (where xampplite is running).
This page provides nice PHP script to download the file back to the client PC once it's been converted:
http://www.zubrag.com/scripts/download.php
With these tools and some basic html and PHP script skills a nice web service can be developed for your home network.
Hope this is found helpful,
Mitch