RobGP is a genetic programming system written from the ground up in C++. It's primary goals are efficiency, ease of use, and extensibility. It's distinguishing feature is that it has a modified version of Koza's architecture altering operations.
The latest release of RobGP is version 0.1:
http://sourceforge.net/projects/robgp/files/robgp/0.1/robgp-0.1.tar.gz/download
The latest subversion revisions often contain many new and exciting features as well as bug fixes and documentation improvements. The system is highly stable and using the latest SVN version is encouraged.