While mobile phones quickly are becoming the primary computing platform for many users, these devices are programmed and used in a manner akin to miniature PCs, each in a silo and interacting primarily with the cloud. We will propose a distributed software platform to break out of this mold and enable capabilities and applications that take advantage of the computing, communication, and sensing capabilities of an ensemble of mobiles, whether in proximity or across a wide area. For instance, a traffic-monitoring application could have a set of mobile nodes in particular locations gather data from their GPS, accelerometer, and microphone sensors, perform custom processing of the sensed data in their local neighborhood to detect unusual traffic conditions, and report the processed data back to a server for aggregation and reporting. Our technology consists of a trusted core that includes services such as localization, communication, and accounting, and a software sandbox for securely executing untrusted application code that builds on these services. We will demo the system in operation and show a couple of applications that illustrate its capabilities: rich traffic monitoring and an on-demand Webcam.