We introduce Impromptu Clusters (ICs), a new abstraction that makes it possible to leverage cloud-based clusters to execute short-lived parallel tasks, for example Internet services that use parallelism to deliver near-interactive responses. ICs are particularly relevant for resource-intensive web applications in areas such as bioinformatics, graphics rendering, computational finance, and search. In an IC, an application encapsulated inside a virtual machine (VM) is swiftly forked into multiple copies that execute on different physical hosts, and then disappear when the computation ends. SnowFlock, our IC prototype, offers near-interactive response times for many highly-parallelizable workloads, achieves subsecond parallel VM clone times, and has negligible runtime overhead.