The Tiny tools for XSI continue to grow. Right now we're up to 4. TinyCurveExtrude and TinyThickness augment XSI's basic extrusion tools, TinyMeshMapper uses Juan Brockhuse's elegant and clever technique for using UVs to deform geometry onto a polygonal mesh and development of TinyCap, which closes holes in meshes with profiled caps, is expecting a more useful release in the coming weeks. There's also a Roadkill plugin for XSI. Roadkill is the open source UV unwrapper originally built into Blender.

All of these tools significantly extend XSI's built in tools. Find out more here