Few things are as unpleasant as dropping a chain onto a new frame's bottom bracket shell, but one that comes to mind is dropping a chain under power, which is like the first one, only extremely painful. The 14g 3rd Eye Chain Watcher adjusts to fit 1-1/8-inch to 1-3/8-inch seat tubes and keeps your chain on your ring.

Keeping your chain in place on a single chainring doesn't get any simpler or more elegant than the 68g Chain Keeper.