I use an Emerson 18 LED rear flasher,
The beast uses 18 red LEDs behind a red reflector and is fed with 4 AA batteries. It can flash, low red or high red output. The thing is aggrivating to the extreme for drivers behind my recumbent bike, they change lanes to stay away from that thing.
It is rated for flashing at 100 hours and 25 hours of bright red steady output. That monster is what made me wish there was bright white LEDs back in 2000... three years later the Mag 2D Madmax+ R2H Luxeon light burns up the night at 3 watts (Brinkmann 2AA BB500 R2H mounted on helmet)
Once you get the light, take those magnets off the back (it is an "electronic safety torch") Slap 4 AA batteries in and watch out! That thing is BRIGHT! I use mine for flash mode with a Trek 5 LED flasher above it as it flashes much faster and with the difference in flash rate, it is much more disturbing to people that see it. Two different flash rates give that freaky light show look much better.
How to mount it? Back in March, 2000... I first removed the magnets off the back to lighten it up. In the corners of the light there are 4 screws, remove them and there will be the reflector lens and gasket to take off. Take two long plastic cable ties and lay them in between the LEDs. Notch the gasket for the ties and screw the 4 screws and lens back in. Click the cable ties together for a rather large loop. You must have a large loop so you can take the battery door off to change the batteries without dissassembling the light and changing out the ties.
Now you have two loops to attach more cable ties to attach them to your rear rack. If you don't have a rack, either attach a small bar through the seat rails to cable tie it to or use your imagination.
To see this light, go here
http://www.sheldonbrown.com/harris/reallite/
To purchase one without the bracket (and a lot less than $40) go here
http://www.jandr.com/JRProductPage.process?RestartFlow=t&
Merchant_Id=1&Section_Id=1&Product_Id=1627454
$5 is a great deal although the shipping costs more than the light! /ubbthreads/images/graemlins/banghead.gif J&R shows the WRONG light so be aware of that... don't forget the little 5 LED blinkie for a much faster blink rate so cars can figure out what distance you are from them. This is not needed if you just run the light without flashing. Mine has been running for over three-and-a-half years with no problems. Picked mine up at Target for $10 at the time.