Allow a "check out" "check in" of files?

Revision control? I'm not being set up am I?

http://en.wikipedia.org/wiki/Revision_control

There are two common ways to do this, on is some sort of version control to contain your files and have users check them in and out when using them, however this involves your users learning version control.

The other is to use something like Sharepoint or other document management systems that will give you a document repository (often online) and the ability to check files in and out. The advantage of Sharepoint is that it integrates well with office if this is something you need.