The Replace ICE String tool lets you effortlessly change attribute names in your ICE trees.

A tool free to download for XSI, that mimics the Tab menu from Houdini and Nuke.

A great way to speed up your workflow.

Soft Effector is a plugin that removes the snap from IK animation. Also provided is a version that works with stretchy bones.

The SaveRenderMesh shader allows you to capture the actual tesselated geometry that mental ray uses to produce the final render and saves it out as an OBJ file.

This means you can create geometry from sub-division surfaces, displacement maps, particle instances, hair, and geometry shaders.

Post Light lets you accurately light your scene from inside XSI’s FX Tree. You can add lights to the scene and in a matter of seconds, Post Light will render an accurate preview of how the final render will appear.

Full documentation after the link.

Taut is a C++ modelling operator for straightening edges. It features an intelligent algorithm that searches your current selection for distinct lines, which allows multiple edges to be straightened at once. It also features three modes of operation that provide different ways of distributing the vertices along the lines.

This tool lets you get a bit more insight into what is happening under the hood of XSI’s rendertree. Using it as a intermediate node, it will display all the data that passes through it in a way similar to how Photoshop Levels is shown. It will also show you the number of times the shader has been evaluated, as well as displaying the minimum and maximum range (which is dead handy for setting up commonly used nodes such as Change Range and Gradient Mixer).