Second the MSFT Optical Wheel mouse. I have three of 'em in use, and two spare just in case for backups (but I've yet to need to replace one). You can actually get them for less than $15. I think I paid an average of like $5 each during various sales/specials/etc. at Staples, Office Max, and the like.
Very well made, simple, and no funky shapes like so many mice out there today.