There are a number of great development tools that integrate well with Unity 3D, like 3D Studio Max and Maya. However, many of these programs can cost thousands of dollars (like 3D Studio Max and Maya). Here are some links to free tools that may help you to develop your games:
Blender: A 3D modeling and animation program, with its own built-in game engine. Blender has a steep, often infuriating, learning curve, but if you buy a book you’ll be fine. I recommend Blender for Dummies.
GIMP: A 2D photo manipulation program, somewhat like Photoshop.
Paint.NET: Another 2D photo manipulation program, with perhaps a simpler interface than GIMP. ...