A completely free version of the professional Softimage|XSI modelling package. Supports Source, CryEngine 2, Unreal Engine 3, XNA, Unity, and more.
The XSI Mod Tool is built on the same powerful XSI 3D engine used by leading game companies like Valve, Lionhead Studios, Pandemic, Konami, CAPCOM, and more. With Mod Tool, you can create original games for PC and Xbox 360 using XNA, or create mods for games like Crysis and Half-Life 2. It's the easiest way to start making games.
The Mod Tool plugs into all the major game engines and development frameworks for next-generation games, casual games, mods for existing titles and even Flash-based 3D games, including:
(There are a few limitations, but nothing major when it comes to modding.)