I'm using a Logitech optiocal 2 button w/scroll wheel at work. Very nice.
At home I'm still using my Kensington Expert Mouse trackball. It's the older serial/ps2 one, not the newer PS2/USB one. I even picked up some replacement balls for it - a "crystal ball" from third party(can't find a link) that looks like it's glowing when the light hits it right and a blue one that Kensignton put out. The blue one reminds me of a bowling ball, the way the colors shimmer when you turn it.