Zotero offers word processing plugins for
OpenOffice (aka LibreOffice)
and now Google Docs!
The plugin adds a Zotero toolbar to your word processor that allows you to add citations to your document while you write.
To add a citation, put your cursor where you want it to go, and click the "Add/Edit Citation" button on the toolbar. Select the reference you want to cite and click OK.
At the end of your paper, click the "Add/Edit Bibliography" button. Your bibliography will appear, and any new citations you add will be added to the bibliography automatically. Change bibliographic styles with the "Document Preferences" button.
It's easy to create a bibliography from your Zotero library:
Or you can just drag and drop references from Zotero into your document! They'll turn into fully formatted citations. This works with any word processor including Google Docs.
To see all of your styles and add additional styles to your Zotero program:
There are currently 2 annotated styles available from the Zotero Style Repository, Chicago and APA.
For the Chicago Annotated style, you put the annotation in the "EXTRA" field in the Zotero record (see screenshots below).
For the APA Annotated style, you put the annotation in the "ABSTRACT" field in the Zotero.
Here are the steps for creating an annotated bibliography in Chicago style.
First, annotate the reference in Zotero.
Select the references you want for the bibliography
Export the bibliography from Zotero using the Chicago Annotated style.
Here is the bibliography with annotations.