Windows only: Collaboration on projects is much easier than it used to be with services like Google Docs, but if you need some of the features in Office itself, free plugin CircleDoc makes real-time collaboration possible in PowerPoint 2007.
Similar to previously mentioned OffiSync, CircleDoc works with Microsoft Office to allow in-program, real-time collaboration on documents. Unlike OffiSync, CircleDoc does not integrate with Google Docs — instead, it stores all of its information on Amazon's S3 servers. Collaborators can invite people to join, chat as they work and immediately sync their changes to the document with the click of a button. The difference between the way OffiSync and CircleDocs handle syncing, it seems, is that CircleDocs streamlines the process by avoiding confusing version conflicts — when you sync your changes, they are immediately applied to the document.
This may seem like data loss waiting to happen, but as long as each person has their own, separate responsibilities (i.e. no one is editing the same slide at the same time as anyone else), it isn't likely going to be a problem. Furthermore, collaborators can also take "snapshots" of the project at any given time. This way, if you make a major change to the document but you end up preferring the older version, you can go back and copy the old slides back into the document. In theory, this seems like an easier way to collaborate — instead of notifications crying for your attention and action every time someone syncs a change, you can just silently save snapshots to go back to later should something happen and everyone else can stay focused on what they're doing.
The other big plus — for right now, at least — is that CircleDocs for Powerpoint 2007 is available as a free beta. There are a few known issues at the moment, but nothing major. (On the chance that you run into them, they are little more than minor inconveniences.) Whether you're looking to check out collaboration software for the first time or you're a dedicated OffiSync user, you can't go wrong by trying it out while it's still free — you may find you prefer the way it fits into your workflow (though if you're beholden to Google Docs, you'll probably want to just stick with OffiSync).
CircleDoc is a free download, Windows Only. Right now it only works with PowerPoint 2007, although they note that it will eventually support 2010. Pricing has yet to be announced for the final version.