I've added a Planet Bike Blinky 5 taillight to my rear rack. Costs about $15-$20 from most sources and runs for many hours on ordinary and readily available AAA alkaline or rechargeable NiMH batteries. It's a good addition to my helmet mounted Blackburn 2'Fer. These lights are equally bright.

The only significantly brighter taillight I've seen locally is the Cygolite Hotshot 80, which costs about twice as much, around $50, and is very bright -- good enough to be seen in daylight.

Along with two separated white front lights, these give drivers a better sense of perspective to quickly determine my approximate distance and speed by providing to light points that converge or diverge, depending on direction.