R is an open source programming language and software environment designed for statistical computing and visualization. R was designed by Ross Ihaka and Robert Gentleman at the University of Auckland, New Zealand beginning in 1993 and is rapidly becoming the go-to tool for statistical analysis of very large data sets. It has been commercialized by a company called Revolution Analytics, which is pursuing a services and support model inspired by Red Hat's support for Linux. R is available under the GNU General Public License.
Other big-data technologies on the list include Cascading, Cassandra, HBase and mongoDB.
CIO.com: 9 open-source technologies to watch