Roadkill is a UV unwrapper heavily based on the unwrapper used in Blender.  Roadkill For XSI is a plugin that interfaces with Roadkill to let you unwrap UVs directly in XSI. Cut lines are marked in XSI by selecting edges prior to unwrapping. You can unwrap UVs with a single click or have XSI have Roadkill start up in GUI mode and unwrap interactively.

Requirements  

XSI with Python installed.

Download

Andy Swann's Roadkill application can be downloaded here

The Roadkill plugin for XSI can be downloaded here.

Thanks to Francis O-Brien and Andy Swann forRoadkill 

Installation

The XSI plugin is a self installing plugin, simply extract the py file from the archive and drop into the application/plugins folder in your user or custom workgroup.

For Roadkill installation, please follow the instructions that come with the Roadkill download

Use

• Select the model you wish to unwrap OR:-
• Select the edges you want to mark as cuts
• From the left hand panel in XSI, select Model->Property->Texture Projection->RoadKill

A property page appears listing the following options

•  Use ABF unwrap (Off = use LSCM)
  ABF (Angle Based Flattening) and LSCM (Least Squares Conformal Map) are different unwrapping methods. According to the Roadkill documentation, ABF seems to work best for organinc meshes, LSCM better for man made objects.
• Fill Holes
• Use Roadkill Interactively.
  By default the plugin uses Roadkill in command line mode; the mesh is loaded by Roadkill, unwrapped and spat back to XSI transparently. If you need more interaction or wish to take advantage of some of Roadkill's features you can launch Roadkill in GUI mode by toggling this option. The mesh is fed back to XSI when you quit Roadkill. No saving need be done.
  Please see the Roadkill Documentation for details regarding the use of Roadkill in Gui mode.

When you've selected the options you need click on OK. The mesh is fed to Roadkill, unwrapped and the new UVs added to the mesh as a new UV set

Roadkill is a one click solution, more or less; there's little room for squeezing the mesh, pulling areas out that need more detail. You can, however, use cuts around areas to produce islands of UVs where you need greater or less detail, blending these back in XSI's Rendertree to get the texel detail where you need it. I'll usually make a number of UV sets for the detail and one large UV set covering the entire mesh that I'll use to drive a blend mask for the fine detail UVs.

Preferences

The plugin uses a custom preference to store the location of the Roadkill executable. The location defaults to C:\program Files\Roadkill1_1. Change this if you've installed Roadkill to a different location.

Credits

Many thanks to Andy Swann for writing Roadkill and Francis O'Brien who, I believe, hosts the Roadkill pages. Thnaks also to the Blender team for the core.