Google Docs now has built-in citation, using the research tool.
The Research tool makes it easy to add information from the web to your documents and presentations. To access the tool:
- Select the Research option from the Tools menu.
- Use the keyboard shortcut (Ctrl+Cmd+I on Mac, Ctrl+Alt+Shift+I on PC).
- Right-click on a specific word and select Research.
- Choose scholar in the search bar; search for your citation, and click on "Cite as Footnote".
Source. EDIT: THIS IS NOW BROKEN. SOURCE
I have been made aware of a third-party add-on that provides more extensive citation functionality to Google Docs called Paperpile. It provides many different inline and footnoted citation styles (They claim >7000).
Also, Faculty of 1000 Workspace (http://f1000.com/work/; geared at life sciences) has a google docs extension (http://f1000.com/work/faq/google-docs-add-on)