Microsoft Comfort Optical Mouse 3000
According to the above review at Tom's Hardware Guide, the ergonomics of this mouse are irreproachable. It is the first corded mouse from Microsoft using their new 1000 dpi high-definition optical technology. Of course, you can get up to 2000 dpi with high-end mice from Logitech and others, but 1000 dpi is more than adequate for playing multi-table poker.
Newegg has this mouse for $20. Combine it with an Icemat Second Edition Mouse Pad for $38, and you have mousing perfection. I have been using this combination for a month, and I really don't know how I ever got along without it. Just be sure to put the little surfing pads that come with the Icemat on the mouse feet, otherwise the plastic feet will scrape noisily on the glass surface of the Icemat.