HUBzero is an open source software platform for building powerful Web sites that support scientific discovery, learning, and collaboration. Some refer to such web sites as "collaboratories" supporting "team science." We call them "hubs" because each site becomes a focal point for its user community. The HUBzero platform now supports dozens of hubs across a variety of disciplines, including cancer research, pharmaceuticals, biofuels, microelectromechanical systems, climate modeling, water quality, volcanology, and more.