Windows: Google Cloud Connect for Microsoft Office automatically syncs your Office documents between your desktop and Google Docs' cloud storage, allowing you to seamlessly collaborate with other Google users and back up all your Office files.
The first thing to know about Google Cloud Connect is that it doesn't allow you to edit the synced Office docs from Google Docs. Instead, Google Docs is simply acting as the backend for storing, syncing, and collaborating from directly in Microsoft Office applications. So don't think of Cloud Connect as a tool that allows you to use both Google Docs and Microsoft Office interchangeably. Instead, think of it as a tool that makes Microsoft Office more Docs-like.
It brings Docs' revision history, easy collaboration and sharing to Office. And while it's not necessarily the best-case scenario, it's not bad.
Cloud Connect is currently available on Windows only for early testers; you can sign up here.