This little tail light mounts quickly to your helmet, seatpost, handlebars, head tube, mop handle... Limited only by the reach of the Velcro straps.
(You thought that was going to say, Limited only by the reach of your imagination, didn't you?)
One LED provides 200 degrees of visibility.
See also Spok's forward-facing friend, the Spok Micro Bike Headlight.
Save when you buy a headlight and tail light set: Planet Bike Spok LED Light Set.