I use Blender now and then, but it can take a while to get used to in my experience. XSI mod tools is nice too, good interface.
Also, there's a free non-commercial Maya that you can get. Autodesk - Autodesk Maya - Personal Learning Edition
I haven't used it, but Maya seems like a nice program.
I didn't make it!
XSI Is probably the easiest 3d modeling tool to pick up, It's also one of the most affordable starting at only 500usd. Or, if you just want to learn 3d modeling and and make modifications to games for free you can get XSI mod tools for free.