Surround SCM has all the goodies that one would normally associate with a source control tool. (Atomic check-in's, branching, diffing, merging, blah blah blah.) It also has some added features for document revisioning, particularly MS Word docs, that could be very helpful. It is actually integrated into Word so that check out's and check in's can be done without ever leaving the word processor. It also supports diffing of Word documents which I know we use a lot for all the reams of documentation we create.
There is both a full GUI client and a command line client and it comes on Windows/Mac OS X/Linux/Solaris. (For the curious it is written with Qt.) It might be worth checking out.