well the BEST program out there for creating a sing would have to be 3ds max. since all you need to make a sing is two boxes you could point and click and prolly make the shape in a few seconds. then you could easly apply a texture thru the simple and upfront material editor. But speeking more realistically. The best that you can get for free is anim8or, you can find it at
www.anim8or.com. they have a somwhat symple free program there that will alow you to make the sign of your dreams!