This topic has been discussed extensively. I recommend a forum search. Use google, as the forum's search function is oddly hit and miss.

Check out ebay and amazon for cree headlights. These typically run around $15 to $20 with battery, a couple of rubber band mounts and an ac charger. If you can plug in an ac charger at work somewhere this might be a great option for you. Most of these cree lights have a blinking function, but it varies.

I also use a cree flashlight as a headlight on my helmet. A bit heavy, but functional and very bright. The batteries use an ac charger as well.

If you can use an ac charger, you can save a ton of money and get basically the brightest headlight on the market.