Hello: The best light BY FAR for commuting is a LIGHTMAN xenon strobe. Visible for up to 3 miles, these strobes are three times stronger than LED “blinkies”. They are very easy to mount on the back of any bike. I put two with clear lenses on the back of my RANS Rocket by using the belt clip attachment -- they slip right on the back of the seat, and they stay on! There are NO brighter lights available for cycling safety. These blink 80x/min. Great lights. Available in variety of colors: clear, red, amber, blue. They retail for $25 (plus $5 s/h) at the Lightman website, but you can get them much cheaper on eBay. Here's a link if you want to take a look.
http://cgi.ebay.com/ws/eBayISAPI.dll...MESE%3AIT&rd=1
GOOD LUCK, and happy cycling! Todd