I am looking for a Mathematica package that can manipulate tensors for supergravity, string theory or M-theory. I am particularly looking for a package that can do spinor and Clifford algebra computations. Also, I would like this package to be able to do wedge and hodge dual, and other computation relating to forms. Can anyone suggest a specific one? I looked for atlas2, but it seems I have to pay to use it without a trial version.
As far as I know there is no package that can do spinor computations in arbitrary dimensions. There is Gamma but that 'only' does gamma matrix computations.
Speaking of xAct, its author maintains a fairly exhaustive list of tensor algebra software here.