Windows/Mac/Linux (Java): Data Crow doesn't just catalogue your movies with IMDB look-ups, organise your MP3s and CDs with discographies, or handle pictures. It handles all three, plus contacts, software, books and much more, and runs on any system.
Data Crow is huge, and can be very intimidating on first boot-up — there are menus upon menus, and it's not immediately obvious how you drop your stuff into this app. Thankfully, there's a beginner's wizard that pops up on first using the app — which can be re-launched from the Help menu if you lose track of it — that walks you through adding, tagging and managing your media.
Once you get the hang of adding your stuff, you'll start seeing some of the cross-indexing and search powers that Data Crow provides. There are lots of web hook-ins to help you grab cover art and details for your media off Amazon.com and other online sources, and a good number of import tools to save you time if you've already done some cataloguing in another app.
The drawbacks to Data Crow, beyond its kitchen sink interface, are that it's a Java app, so it's not tightly integrated with your OS look and feel, and isn't the snappiest app you'll find. That said, if you like the idea of One Media Database to Rule Them All, Data Crow seems like the most obvious contender.
Data Crow is a free download, requires a Java installation to work properly. Windows 7 users, try running the app in Windows XP compatibility mode if you have problems with the app not launching after clicking to start it up.
See something in Data Crow that's a clever feature? Found a way to import and manage media easier or faster? Tell us about it in the comments, because there's a lot to learn.