1. A system comprising: a data processing apparatus that includes a computer-readable storage medium storing executable instructions that cause the data processing apparatus to: obtain a description of one or more computing resource requirements for execution of an application associated with a publicly available service; determine respective amounts of time involved for different private computing devices to obtain software to run the application, the different private computing devices being external to, and separate from, the publicly available service; considering the respective amounts of time involved for the different private computing devices to obtain the software to run the application, match the one or more computing resource requirements to temporarily available computing resources of a selected private computing device to run the software; and cause the selected private computing device to obtain the software and host the application.