OpenPathSampling (OPS) makes it easy to perform many variants of transition path sampling (TPS) and transition interface sampling (TIS), as well as other useful calculations for rare events, such as committor analysis and flux calculations. In addition, it is a powerful library to build new path sampling methods.
OPS is independent of the underlying molecular dynamics engine, and currently has support for OpenMM and Gromacs, as well as an internal engine suitable for 2D toy models.