Blackburn flea.

http://www.blackburndesign.com/lights.html

Mounts with a velcro strap. Easy on and off, super duper lightweight. Can be switched between bikes in about 10 seconds. Rechargeable with ANY 1.5V battery. Waterproof. Lifetime warranty. Very bright. Can be found for about $20 a piece. Highly recommended.

PB superflash is good too.