Microsoft Office documents collaboration - Open Source alternative

WSS (Windows SharePoint Services) is free... well, included with a server license anyway. It will take care of the shared editing problem though it is certainly not open source. I haven't seen any products that offer the same tight integration with Office (Word,Excel,Powerpoint) for web collaboration yet.

Google Apps? Depends on what your needs are. If your spreadsheets are complicated then no, but if you're dealing with simple stuff it may be an option.

I guess it also depends on your office size. In the office of 75 I manage we use Alfresco extensively. We find it easy enough to do just simple revisions all the way through to drop in replacement for SharePoint, document workflow and wiki.

We primarily use it via samba or webdav, but also use it remotely via the web interface and use the permissions for giving customers access to files too.

I have found the java engine to be quite a hog to run though and Microsoft domain integration documentation quite lacking at times.

Care to define your requirements any better?