I like an ARC-P for my key chain but I think it was almost $40, super bright, awesome key chain light VERY RIGID and bright.

For $10 I would say this:
http://www.amazon.com/Super-Bright-Heavy...3685&sr=8-1

I have one very similar I got free with an order from a parts company it's lasted over a year and I've sent it through the wash, and dryer, and it still works. It is one of the two lights that stays by my bed. The other is a SureFire G3 w/LED conversion and ~130 lumens I believe.

It really depends what you want it for, etc.