i bought two ultrafire wf-501s on deal extreme and one drop in module with 5 different modes. one i run on solid with a diffuser lense and the other flashes sos(or strobe in thick fog/heavy rain). the whole thing was under 35 dollars, i had batteries and charger and mounts already. i would perfer different flashing modes, but its cheap and easy on/off on bike and its bloody bright from quite the distance. not like the pbsf, which is bright right infront of you but a few hunderad meters and its fairly dim. imo this current setup is a bit overboard.

http://www.dealextreme.com/details.dx/sku.20333