Some of these little LED keychain lights are actually very good and are virtually disposable items at $0.45 each.
http://www.dealextreme.com/details.dx/sku.1100They are very useful items to attach to tent zippers, pocket and bag zippers etc and forgotten about.
+1. I bought ten for $4.50 with free shipping. The first day I had 'em I left one in my pants pocket and accidentally ran it thru the washing machine, but it still worked!

All ten worked fine and they're surprisingly bright. You may not want to trust your life to a 45 cent light but they're great for general use. I keep one on my keyring and a couple on various zipper pulls. So far none have failed.