I bought one of these:
http://www.amazon.co.uk/MIMO-UM-710-Powered-Swivel-Display/dp/B001L1R9LK
which i keep off to the side of my two main monitors at work. I keep various system-monitoring utilities on it, so they are always visible without getting in the way of my main work area.
Do I need it? No - TBH, I could quite easily live without it. But it's handy, and it was fairly cheap, so why not make my life just that little bit easier?