In my work we always start before daylight. Between that and staying out after dark fishing, it's a rare day that I don't have one of these on my belt all day long.

http://www.maglite.com/AA_Cell_LED.asp

I think it was around $25, maybe a little less.

On overcast afternoons when I ride home I strap it to the handlebars with a bungee loop and run it in flash mode. From the actions of motorists I seem to be more noticeable on overcast days running this light than I am on bright, sunny days that I don't use it.