The API forms a fundamental part of several projects, including:
The online library view at zotero.org, which is built using the same API
Zotpress, a WordPress plugin for including citations to items in your Zotero library in blog posts. The plugin makes extensive use of the read API and implements both key-based and OAuth for access control.
Biblio Bouts, an game where participants compete to collect high quality sources online. The game uses the API to see what items participants have saved.
phpZoteroWebDAV, a WebDAV implementation in PHP which allows users to sync their attachments to their own webservers, including an online library and attachment view building off the read API. (AGPL)
Scanner for Zotero, Android app which saves items to Zotero libraries using the write API (Java, GPL-licensed)
Zandy Android app using the read and write APIs to provide full access to Zotero libraries (Java, AGPL-licensed)
ZotPad iPad/iPhone app using the read and file upload APIs to provide read access to Zotero libraries and read/write access to attachment files. Write access is planned. (Objective C, GPL-licensed)