Softimage Mod Tool
XSI is the program used to make models for Source. There are three commercial versions, XSI Foundation, Essentials, and Advanced.
XSI Mod Tool
The XSI Mod Tool 6 is a completely free version of the professional XSI modelling package. It has most of the features of the professional versions (certainly everything you'd need for mods) and allows you to export your work to Source without restriction.
ValveSource addon
The Mod Tool is used with the ValveSource addon, which is required to export your work to a format Source's model compiler can understand.

Valve Source Add-On for XSI5.11/6.0 users
The Valve Source Add-on that was included in the SDK has problems with XSI 5.0/5.11/6.0. Noesis Interactive has released two updated Add-ons for XSI 5.11/6.0 users.
Valve Source Add-On for XSI 5.11/6.0
Tutorials
Generic XSI Tutorials
Quickstart Guide for Max/Maya/Lightwave users
- MAX to XSI Jumpstart
- DISCREET 3DS MAX TO SOFTIMAGE XSI TRANSITION TIPS
- Maya 2 XSI Quickstart Guide
- Lightwave 2 XSI Quick Start(pdf)
Source Tutorials
- Model Creation Overview
- Compiling Models
- Character Creation Overview
- Viewmodels in XSI
- Replacing HL2DM player models with XSI Mod Tool
- Converting brush structures in your map into a prop_static with XSI Mod Tool
- Getting Static Props into Source via XSI|EXP for HL2 by Joseph Spataro
Note:File Structure in this tutorial is outdated. Replace
sourcesdk/cstrike_sample_content
withsourcesdk_content/cstrike
. See Source SDK Files and Directory Structure for details.
Video Tutorials
Creating a prop in XSI and Texturing it with Photoshop by FlashJackson - 1+ hour video tutorial that shows prop creation process from modeling, texturing to compiling. (Also On Main Page For Now of Redemtion Site [1]http://redemptionmod.com/index.php Scroll to the bottom.)
Colt 1911 XSI Video Tutorial by desty - 5 hour video tutorial which shows the whole modeling process. Reference pictures and renders file are also available.
3D Buzz - "Introduction to XSI" 5 hour VTM series
Digital-Tutors - XSI free turorials
Samples
SourceSDK contains various character/weapon/vehicle sample scenes (.scn) for XSI. See sourcesdk_content/<game name>/modelsrc/...
You can replace the model to make your own characters/weapons/vehicles.
Also there is Korin XSI Mod Tool Scene data released by nkinta. It shows how to make an Alyx replacement model with full facial animations.