Yeah - it took me a little while to take that step, but I'm getting more and more into a professional development process here for my own game.

The Unity Asset Server is the version control system to be used with Unity. Version control systems are used by developers to track their changes and make sure that the process is smooth even if multiple people are working on the same project simultanuously. So, the main purpose of version control systems is making the software-development process in teams smooth. However, it's also useful in solo-projects because you can make changes, and if you terribly break everyhing just revert to the last version where everything still worked.

Up to now, I was using Subversion (which I have added just a short time ago - before that, I was working naked, kind of), but the Asset Server is perfectly integrated into Unity and can also handle large binary files. Well, if you're interested in the technical details, you might check out the Asset Server Guide on the documentation pages of Unity.

For those who care about Mac OS X Leopard: Starting today, I'm also using Time Machine for incremental backups of everything I do. So, you see: I'm doing what I can to save my assets ;-)