Maya is a popular 3D modeling/animation/rendering suite from Alias. It is a powerful tool for modeling, texturing and animating props and characters. It also contains many advanced features which are not applicable to Source modding (dynamics, rendering, etc.). While Valve favors XSI for modeling and animating, some users may prefer Maya's rich feature set.
A free version of Maya called the Maya Personal Learning Edition is available to all for non-commercial use. It is a feature-complete version of Maya Complete 7 with render watermarks and export restrictions. These constraints do not inhibit Source modding in any way.
Get the Maya Personal Learning Edition for free here.