First introduced in 1968 by computing pioneer Douglas Engelbart, the mouse has evolved from an extremely simplistic "position indicator" to a highly specialized tool relied upon by billions. Although the basic input principles remain unchanged, modern mice offer decades of ergonomic refinement, instantly adjustable tracking sensitivity, macro abilities, customizable LED colors, and there are even touch-based models.
Obviously that development didn't happen overnight and if you're interested, Macworld ran a piece in 2008 highlighting the mouse's history. Before you go, be sure to tell us what your first mouse was (besides the freebies that come with OEM machines) as well as what you're currently using and what features are most important to you. I can't recall my first "real" mouse, but I'm currently using the second-generation Logitech G5.