knog boomer
http://www.amazon.com/Knog-Twinpack-.../dp/B0033I0HT0

sold individually front and back and quite a bright, awesome light for getting noticed fast.
plus it will mount to bars of a number of sizes and you can take them on and off easy.

for those of you that don't know these guys are brighter than a pbsf:
http://www.youtube.com/watch?v=eTBe1Ewn_ds