Even with reasonable overprovisioning, today's Internet application clusters are unable to handle major traffic spikes and flash crowds. As an alternative to fixed-size, dedicated clusters, we propose a dynamically-shared application cluster model based on virtual machines. The system is dubbed "OnCall" for the extra computing capacity that is always on call in case of traffic spikes. OnCall's approach to spike management relies on the use of an economically-efficient marketplace of cluster resources. OnCall works autonomically by allowing applications to trade computing capacity on a free market through the use of automated market policies; the appropriate applications are then automatically activated on the traded nodes. As demonstrated in our prototype implementation, OnCall allows applications to handle spikes while still maintaining inter-application performance isolation and providing useful resource guarantees to all applications on the cluster.