Just previewed the new Google Sites functionality that was released (http://sites.google.com) recently. It is supposedly their attempt to capture some of the collaboration market that MOSS plays in. To be fair, it's early in the lifecycle of Google Sites, but at this point I think it would only be a viable alternative for shops that need a minimal collaborative environment. The UI is very slick however, Microsoft could take a few cues from them in that department (especially the nice cross browser Ajax interfaces, and deadly simple usage patterns).
To try it out, at least for now, you need to have your own domain name to push up some files to verify that you are your own "company". Once you do that, you will be able to create sites, invite contributors, pick from some style templates, share files and documents, create web pages, create announcements, create very rudimentary lists, and create dashboards (which consist of small widgets for other Google apps).
I think given enough time, Google could have a pretty solid product for small business collaboration, but as it stands now, I don't think Enterprise customers will use it. If they come out with a version that companies could install on their own servers behind firewalls, only then could it come close to competing with MOSS.
