I created a simple Ebook catalogue application for my own use, but decided to make it available, for free of course.
I'm willing to expand it (time permitting) to meet any specific features you think would be cool.
You basically tell it the folder locations where your Ebooks are kept, and it will look in all sub-folders for matching Ebooks (it supports extensions of PDF, CHM, LIT, DOC, HTM, HTML, RTF, TXT, DJV, DJVU, PDB and PRC at the moment but I can easily add additional extension support).
Whenever you get new Ebooks, just start the "Update Library" process and the new books will be picked up and added to the library.
If you know the ISBN of your E-book (or the corresponding print edition), then it will also download the cover-art and author/publisher details from my server.
I'm also currently working on a plugin system (very early stages) that will allow third party developers to develop plugins. For example, to export a selected book to your hardware reader, or to generate a text list/csv of your Ebooks.
The software runs on Windows XP only and requires the Microsoft .NET framework 1.1 or greater to be installed.
You can find it at http://www.myebooklibrary.co.uk