I'm a huge user of Google products for personal and work uses. I'm also someone who likes to have my own backup of data. There are also people who want to take their data with them if they are leaving Google for other services.
Google Takeout lets you download your data from a variety of Google's services to your computer. You can do all of your data or select the services to download data from. Services include +1's, Buzz, Circles, Contacts, Docs, Knol, Picasa Web Albums, your Profile, your Google+ Stream, and Google Voice.
You select "all", or individually select services, and then click "Create Archive" and it will download a zip file of your data. The time and size will obviously depend on how much data you have.
Security Note: Since Google Takeout involves your personal information, we'll sometimes take extra care to protect you by asking you to verify your password even though you're already signed in.You can also export your Docs in Docs, by selecting the documents you want, click "more", then "download", "all items" and the formats you want them downloaded. You can also use GDoc backup which will backup all of your Docs to your computer.
Notice that blogs are not included in the list above. To download a copy of your Blogger blog, go to "settings", "other" "export blog" and export it. You also have the ability to export your contacts, bookmarks, and more from each application's setting menu.
Related:
Google for Educators Resources
Why I Use Google's Products as an Educator