The source code and binaries of the learning system CORElearn can be downloaded as:
The system supports decision and regression trees as well as improved random forests and OrdEval algorithm. Supported data input formats are C4.5, C5, and native format. The system is tested to compile and run on Windows and Linux. The documentation is available through R, just install the package and type ?CORElearn at R prompt.
CORElearn is available under the GNU public license (GPLv3 or later).
Other licensing might be possible.
Some of the functionality and features are described in my papers. The R package contains the documentation which applies also to the standalone version.