From its website :

“Cactus is an open source problem solving environment designed for scientists and engineers. Its modular structure easily enables parallel computation across different architectures and collaborative code development between different groups… Cactus provides easy access to many cutting edge software technologies being developed in the academic research community, including the Globus Metacomputing Toolkit, HDF5 parallel file I/O, the PETSc scientific library, adaptive mesh refinement, web interfaces, and advanced visualization tools.

The Cactus user community has created and is maintaining toolkits for several research fields. The Einstein Toolkit addresses computational relativistic astrophysics, supporting simulations of black holes, neutron stars, and related systems.”