When Zotero senses items on a web page, it adds an icon in the Firefox's location bar. A user can click on the icon and Zotero saves the reference information in one of the user's libraries. BibSonomy now offers the icon for Zotero users. The icon is shown on pages with BibTeX entries.
By clicking on this icon, a pop up allows you to select the entries you want to export. Click OK and the selected references will appear in your Zotero library.
The other way around is not fully automized yet. However, there is a nice blog entry which shows how to easily copy and paste Zotero data into a BibSonomy "postPublication" snippet.
Translated from this tutorial: Navigate to Zotero's settings-tabs and select the export tab. Push the plus button below the "Site Specific Settings" area to add a new setting. In the new pop up, enter "bibsonomy.org" and select "BibTeX" as output format. You can confirm your new entry by clicking on OK.
When you log in to BibSonomy and click on "postPublication", you can drag and drop a Zotero entry into the box below "Insert your publication snippet here". Enter save and the BibTex entry will be displayed in the appropriate fields.
