Weka is a data mining package which includes a wide variety of methods. Its easy to use interface makes it accessible for general use, while its flexibility and extensibility make it suitable for academic use.
Weka is written in Java and released under the GNU General Public Licence (GPL). It has an API which allows the algorithms to be called from other programs.
Weka can be run on Windows, Linux, Mac and other platforms.
Other programs such as Rapid Miner and R can access Weka routines.