Usage of shared resources can be managed by enabling users to obtain different types of guarantees at different times for various types and/or levels of resource capacity. A user can select to have an amount or rate of capacity dedicated to that user. A user can also select reserved capacity for at least a portion of the requests, tasks, or program execution for that user, where the user has priority to that capacity but other users can utilize the excess capacity during other periods. Users can alternatively specify to use the excess capacity or other variable, non-guaranteed capacity. The capacity can be for any appropriate functional aspect of a resource, such as computational capacity, throughput, latency, bandwidth, and storage. Users can submit bids for various types and combinations of excess capacity, and winning bids can receive dedicated use of the excess capacity for at least a period of time.