Some time ago, we had a two-player game whose most natural input method was the mouse. That’s too bad, because most OSes make it exceedingly difficult to handle two mice separately; they just clump the mice together, so both mice control the same cursor.
Today I stumbled upon a teeny little cross-platform library called manymouse, which handles different mice separately! The interface is really easy (a little low-level, but workable); I got it incorporated hackishly into our SDL sword game in about twenty minutes. So, that’s cool.