[Editor's Note: The Google Desktop API is no longer available. This profile is maintained for historical and research purposes only.]
This is Windows-based client SDK that can also integrate with online content. It uses a Windows COM programming model. From their site: Google Desktop indexes, caches, and searches content on your computer. Its Sidebar provides constant data display from multiple information sources. Through the Google Desktop Software Development Kit's APIs, your applications can send content to the Desktop engine for indexing and storage, send search queries directly to Desktop, define new Sidebar gadgets, communicate between different users' Sidebar gadgets, redefine Desktop user interface actions, and define new Desktop events.