Explore the Zimbra Community and Gallery
Zimbra has a large open source Community that can provide guidance on how to extend Zimbra functionality as well share code based on Zimlets that have been contributed to the Community. An excellent resource for more information is the Zimbra Forums where there are areas dedicated to development and Zimlets.
Another excellent resource for both finding and sharing Zimlets and other custom Zimbra add-ons is the Zimbra Gallery. Zimbra customers and Community members often place their finished creations here for the Community to leverage for their own deployments.