1. A computer-implemented method of managing shared resources, comprising: under control of one or more computer systems configured with executable instructions, receiving a virtual machine instance request associated with a user, the virtual machine instance request specifying a type of virtual machine instance capacity to be used in fulfilling the virtual machine instance request and a target amount of latency for the specified type of virtual machine instance capacity to operate in accordance with while fulfilling the virtual machine instance request; if the type of virtual machine instance capacity is a dedicated capacity type and dedicated capacity for the user capable of providing the target amount of latency is available, fulfilling the virtual machine instance request using the dedicated capacity; if the type of virtual machine instance capacity is a reserved capacity type and reserved capacity for the user capable of providing the target amount of latency is available, fulfilling the virtual machine instance request using the reserved capacity; if the type of virtual machine instance capacity is an excess capacity type: determining whether a bid price is a winning bid price, the winning bid price being greater than other pending bids for the same excess capacity and being at least equal to a market price; and if the bid price is the winning bid price and excess capacity for the user capable of providing the target amount of latency is available, fulfilling the virtual machine instance request using the excess capacity for at least a minimum period of time; and if the type of virtual machine instance capacity is a variable capacity type and variable capacity is available, fulfilling the virtual machine instance request using the variable capacity, the variable capacity capable of having less than the target amount of latency for fulfilling the virtual machine instance request.