I had a custom shift knob made by koolknobs.com. Bruce, who runs it, was really nice and patiently explained all my options. I ended up with a upside-down, bell-shaped smiley knob that was yellow with silver and pearl swirls. I just love it, but it's only 2" in diameter at the top (the widest point). I have small hands so it fits me perfectly, but anyone taller than five feet might have issues.
You can even get personal stuff embedded in them, like pins or buttons. Pretty neat. He has a bunch of different shapes and colors, smiley or not smiley. I think he even has some with lights in them.