Amit Agarwal has a nice blog post explaining how to download all your documents from Google Docs as a zip. If you want to have a back up of your Google Docs online, you can do so using Zoho Docs. Zoho Docs allows uploading of zip files and you can upload the zip file downloaded from Google to Zoho Docs. The slide show below shows the steps involved in backing up your Google Docs in Zoho Docs.
Recently we announced the integration between Google Apps and Zoho Projects. That integration allows you to attach a Google Docs file to the Documents module of Zoho Projects. We are coming up with more such exciting integrations soon. Stay tuned.