Digital lecturing systems (computer and projector, often with PowerPoint) offer physics instructors the ability to incorporate graphics and the power to share and reuse materials. But these systems do a poor job of supporting interaction in the classroom. For instance, with digital presentation systems, instructors have limited ability to spontaneously respond to student questions. This limitation is especially acute during classroom activities such as problem solving, Peer Instruction, and Interactive Lecture Demonstrations (ILDs). A Tablet PC, a laptop computer with a stylus that can be used to "write" on the screen, provides a way for instructors to add digital ink spontaneously to a presentation in progress. The Tablet PC can be a powerful tool for teaching, especially when combined with software systems specifically designed to leverage digital ink for pedagogical uses. Ubiquitous Presenter (UP) is one such freely available system. Developed at the University of California, San Diego, and based on Classroom Presenter, UP allows the instructor to ink prepared digital material (such as exported PowerPoint slides) in real time in class. Ink is automatically archived stroke by stroke and can be reviewed through a web browser (by both students and instructors). The system also supports spontaneous in-class interaction through a web interface--students with web-enabled devices (Tablet PCs, regular laptops, PDAs, and cell phones) can make text-, ink-, or image-based submissions on the instructor's slides. The instructor can review and then project submitted slides to the class and add additional ink, so that material generated by students can be a focus for discussion. A brief video showing UP in action is at http://physics.csusm.edu/UP. In this article, we describe UP and give examples of how UP can support the physics classroom.