Traditional collaborative applications, such as email and newsgroups, are among the most successful and widely distributed applications. However, such services are almost exclusively based on centralized servers, which inherently limits their scalability and fault tolerance. This thesis presents the design of POST, a platform for such collaborative applications which is completely decentralized and is based on the Pastry peer-to-peer overlay. To show that POST is sufficient to support even the most demanding applications, this thesis also presents ePOST, an email service built on POST. ePOST is in use as the primary email system for actual users, demonstrating an email system which is inherently more scalable and potentially more fault tolerant than existing systems. The success of POST shows that peer-to-peer technology is mature enough to support reliable applications, and that other traditionally client-server applications can be improved through the use of peer-to-peer technologies.