Like Costas and Klattman, I would highly recommend the Tim Wegner Mouse. I've got the original Spyderco Mouse and I love it, but its just too expensive for me to carry. My wife got me the new Mouse-Lite for Christmas and it has hardly left my side since. Great, tough little package and a price that can't be beat.
http://blade-tech.com/frontend?command=ProductMatrix&iProductId=51226&iSectionId=999
Flinx
http://blade-tech.com/frontend?command=ProductMatrix&iProductId=51226&iSectionId=999
Flinx