It is called clustering, almost all "supercomputers" are clusters.
But just because you cluster 15 computers with 2 GHz CPUs doesn't mean you now have a 30 GHz general-purpose computer. Only software written with parallelism in mind can actually take advantage of a cluster. Because of this, clusters are only able to reach their maximum efficiency with extremely specialized software (I.E. the software would generally be written specifically for that cluster and the task it is working on).
If you want to learn more about it, you would do well to start on the Beowulf