Nothing fancy here but the Blackburn Mars 2.0 is a pretty bright tail light. The shape of the Mars is unusual and it reminds me of the Martians in the George Pal version of War of the Worlds. Four bright red LEDs shine out the back and two yellow LEDs shine out the sides and the Mars can be set to flash, solid or chase modes. The light comes with both a seat clamp and a clip that can be used to attach the light to a backpack or messenger bag. The triangular shape makes it ideal to mount inside a reflective yellow triangle like the one made by Nathan. I use zip-ties to hold the Mars 2.0 inside the triangle and the triangle and light are zip-tied to the rear of my backpack.
The Mars 2.0 comes with 2 AAA alkaline batteries which Blackburn claims will run the light for 60 hours in solid mode and 200 hours in flash. The light seems to be reasonably well sealed against water but changing the batteries is a fiddly affair involving the removal of four philips head screws. The light sells for about $12 or $13.