I just got back from Wal*Mart where I purchased a cordless optical mouse. It's amazing what you don't realize you need until you get it. The tracking is far superior to the old ball mouse we had been using. The optical mouse that came with my iMac died over a year ago. Actually, it seems that the cable from the mouse to the computer got pulled loose a little. My wife taped it up with clear packing tape, and my daughters use it on their iMac with no problems.
Now, we just have to keep the girls from running off with our cordless one and hiding it where we can't find it.