Microsoft makes a series of wireless mice for laptops. The mouse stores a small device that can be removed and plugged into the USB port on your PC. It's somewhat smaller than normal desktop mice and I find it works well for travel when you'd rather use a mouse on your hotel desk than the pointing stick or touchpad.

I have the optical version and it works fine with my Compac Presario. Fry's often has them at a good price as well.