I just saw this great idea in a letter to the editor in Bicycling magazine:
"Next time you upgrade your equipment, carry the old light in your jersey pocket or car -- and help someone else get home safely. There are plenty of folks who commute on bikes, often at non-daylight hours, and can't afford lights. On one morning ride, I passed a guy riding to work. He asked me where he could get a taillight like mine. I passed the same guy on my way home, and thought about an unused light I had. I went home, got it and caught up to him. He put it to use immediately."