On the desktop, users are accustomed to having visible handles to objects that they can organize, share, and manipulate. Web applications today feature many loosely defined classes of such objects, like flight itineraries, products for sale, people, recipes, and restaurants, but there are no interoperable handles for these high-level semantic objects. On the web, users need visible handles that can represent an evolving set of semantically rich objects. Such handles would enable a simple, direct, and consistent interface for data representation and transfer. This thesis proposes Clui, a platform for exploring a new data type, called a Webit, that provides uniform handles to objects. Users drag and drop Webits between sites to transfer data, auto-fill search forms, map associated locations, or share Webits with others. While Clui offers a developer API to add Webit support to web sites, Clui plugins allow users to use Webits immediately. Plugins create Webits by extracting semantic data from existing web pages, and they augment sites with drag and drop targets that accept and interpret Webits, all without requiring the cooperation of site developers. Contributions of this thesis include design principles, derived from experimentation, that guide the functionality and behavior of handles for web objects; a system design that provides an adoption path for such handles; and a scalable approach for realizing handles that enforce access controls. To evaluate the usability of Webits, we conducted two in-laboratory studies and collected qualitative observations and feedback. The results suggest that the system is usable and effective in improving user efficiency. While using the system, participants expressed enthusiasm and delight, and believed that Webits would be useful for their daily web activities.