As a dedicated "flashaholic" I have many small high end EDC lights but few are really suited for key chain use until recently with the release of the Nitecore Tube. At .34 oz.(9.6gr.), it's a thin, somewhat flat, rectangular USB rechargable 2-mode light (1 and 45 lumens) and costs less than $10. It's like a quality button cell light that never needs batteries...if "small and light weight" are important to you, do yourself a favor and check it out. Hell, for $10, you can give it away if you don't like it.
I got a couple of the Tubes when they were on sale before Christmas. So far I'm quite impressed. I can't really think of much I'd want to change about it. Small, but fits well in the hand. Simple interface. Well put together (with actual screws, no less!). Then you throw in the rechargability? They seem like a heck of a value to me and are my strong recommendation for a keychain size light right now...