HeuristicLab has a powerful implementation of Genetic Programming. It includes problems such as Symbolic Regression, Symbolic Classification, Time Series, Santa Fe Ant Trail, and there is a tutorial to implement custom problems such as the Lawn Mower (which is similar to the Santa Fe Ant Trail). HeuristicLab is implemented in C# and runs on Windows. It's released under GPL and can be freely downloaded.
The implementation of GP is very flexible and extensible, but also performance optimized using online calculations to avoid array allocation and memory overheads. We do include several benchmark problem instances for symbolic regression and classification. There are also more algorithms available such as Random Forests, Neural Networks, k-NN, SVM (if you're doing regression or classification).