Cloud9 is a MapReduce library for Hadoop designed to serve as both a teaching tool and to support research in data-intensive text processing. MapReduce is a programming model for expressing distributed computations on massive datasets and an execution framework for large-scale data processing on clusters of commodity servers. Hadoop provides an open-source implementation of the programming model. The library itself is available on github and distributed under the Apache License.
For additional details on MapReduce algorithm design, Data-Intensive Text Processing with MapReduce by Lin and Dyer is a good resource. This library also serves as a repository of many examples discussed in the book.