We saw a preview of it a little while ago, and now it's here in beta form: MMOUI Minion, the addon updater from the people behind WoWInterface. It's written in Java, and runs on Mac OS X, Linux, and Windows, provided you have an up-to-date Java runtime installed. For Mac OS X users, the web site might warn you about your Java version, but if you've been keeping up with software updates, you should be fine.
As this is a beta, I won't be too hard on it, but I wouldn't recommend using it in its current state. It does show promise, especially with its extensibility (other sites can write plugins to allow Minion to download content from them). I ran into all kinds of problems, from the installer making it difficult to specify a path to the updater maxing out my CPU while idle to it detecting almost all my addons as out-of-date, even ones that aren't, but again, it's beta. The whole reason for a beta phase is to get the software out into the wild for wider debugging, so bugs are to be expected.
Overall, I'd say this could develop into something nice, but isn't quite ready yet. The icon, on the other hand, is totally adorable. If you do want to try it out, you would do well to follow their suggestion of backing up your Interface and WTF folders. I am excited to see what it will look like with more polish.