Gmax is the free software provided by (formerly Discreet, now Autodesk).
It's max's little brother. Same as 3dsmax and Viz, but no render engine. You'll only be able to model and see it in the viewport.
Maya Personal Learning Edition is free. It's a full blown version of Maya but has a limit on the image size you can render out, and it watermarks renderings.
The free version of Softimage XSI is called the XSI mod tool. It's specialized for modding Half Life 2, but I'm sure it's a reasonable facsimile to real XSI.