For a keychain or even an EDC pocket light, I really think the photon freedom is unrivaled. It's tiny, bright, very durable, long lasting, and has dimming and other modes that can be useful without being too complicated to use. For less than $20, I don't think you'll find a better value for an EDC light.

You can get them here for $20 with shipping: http://www.photonlight.com/

Or here for $13 plus shipping, I think it comes to about $17: http://alphanet.zoovy.com/category/photon_microlights_by_lri.photonfreedom/