Cloud systems differ fundamentally in how they offer and charge for resources. While some systems provide a generic programming abstraction at coarse granularity, e.g., a virtual machine rented by the hour, others offer specialized abstractions with fine-grained accounting on a per-request basis. In this paper, we explore Tasklets, an abstraction for instances of short-duration, generic computations that migrate from a host requiring computation to hosts that are willing to provide computation. Tasklets enable fine-grained accounting of resource usage, enabling us to build infrastructure that supports trading computing resources according to various economic models. This computation model is especially attractive in settings where mobile devices can utilize resources in the cloud to mitigate local resource constraints.