I just finished a sheet of SAGE where given a metric, the Christoffel symbols, the Riemann tensor and the Ricci tensor are calculated. I publish it in my google site (at the end of the page).
It is a very rudimentary sheet, but it could be a tiny part of a class for computing GR tensors (or even more) 🙂
Much more!!!!Please! feel free to post either here or in sage-devel google group about possible improvements or your own implementations.