Dojo Foundation Maqetta
Derived from an IBM initiative, Dojo Foundation's Maqetta is an open source tool for building HTML5 applications on desktop and mobile platforms. Visual authoring of HTML5 interfaces is enabled in a browser. User experience designers can assemble live UI mockups via drag-and-drop. UIs built by Maqetta are Web apps that can be handed off from designers to developers.
Maqetta is designed to preserve visual assets across the development lifecycle, and users can plug in arbitrary widget libraries and CSS themes. Maqetta source code can be downloaded and installed on a user's own server. Code can be customized, and contributions to the project itself can be submitted. Maqetta is currently in a preview phase, with a 1.0 release eyed for the coming months.