For travel, I like the Microsoft trackball. It's bigger than a mouse, but once you settle it somewhere, it doesn't have to move and I find it more convenient than a mouse. I can use it in my lap, or even rest it on the front of the keyboard. Setting OSX to ignore the trackpad when the mouse is present prevents those strange lost mouse pointer incidents too.
That trackball is designed for thumb on ball, fingers on buttons, and the extra two buttons work in OSX (but not Parallels) and can be programmed. In Parallels, it's a three button (counting the one on the wheel) mouse; in OSX it's five.
There are also trackballs available designed for index finger on the ball which have the advantage of being usable with either hand.
You will probably have a strong preference for one design or the other, so try both if you can. Someone I work with prefers the marble mouse (finger on the ball) and I prefer to use my thumb and when we work together, we keep switching which device is plugged in because we find each other's rodents awkward.
There is also a model available with a large ball in the center surrounded by buttons that may or may not work well in Parallels but works fine in OSX. I don't like it but a friend loves it.
There are some "travel" mice available, but I have large hands and find them awkwardly small.
Last edited: Nov 17, 2006