Source: http://lifehacker.com/5637067/insync-keeps-your-files-and-google-docs-synced-and-backed-up
Insync creates an always-in-sync folder on your computer that copies files to the cloud and to other machines running Insync. It also syncs your Google Docs to this folder as well, which is pretty great. If anyone shares a Google Doc with you it'll show up in your Insync folder, on your Insync page, and (of course) in Google Docs:
When you make changes locally, those changes are synced back up to Google Docs almost instantly. If you want to keep an eye on anything and everything Insync is up to, you can check in the menubar (system tray in Windows):
While syncing locally to Google Docs is pretty much instant, syncing from Google Docs to your local machine can sometimes take a minute or two—but speed improvements are already underway.
Revisions are tracked with your synced docs, too, so a quick right click can let you look back on all the revisions you've made. Here it is in action:
Insync isn't just for Google Docs, though. In addition to syncing your Gmail contacts, you can also drop any file into the Insync folder and it'll save it to your cloud storage and sync it with your other machines.
Insync is free for individual users and business accounts (limited to five users). Both account types come with 1GB of free storage. Additional storage options are forthcoming.