I bought and sold/gifted many lights in this class. The best IMO is the Liteflux LF2X, a AAA twisty
http://www.dealextreme.com/details.dx/sku.12062 It is about as long as Jayrob's pocket mini (judging from pics) but is thinner.
If you use a 10440 lithium rechargeable it will really blaze. Completely programmable with 2-3 brightness modes from VERY low to VERY high, strobes, etc. Actually the programming interface is way too complicated but once you do it to your liking you can forget it. Mine is really easy to use one-handed and has lived unnoticed in my left front pocket for over a year now...I use it daily (with my off hand I am a righty and that is where the knife lives). This is about my 7th or 8th EDC light and finally found the perfect one for me...superbright and small. And at $53 won't totally break the bank. $53 might seem like a lot but for this brightness and convenience I haven't found better value. No they don't pay me but maybe they should...
The included diffuser (a glorified chapstick cap
) is more useful than I thought and doesn't add too much length. Mine didn't fit great but a rotary tool fixed that problem quickly. I use the diffuser 9 out of 10 times, so I leave it on the end of the light. If I want a spotlight I just pop it off as I grab it out of my pocket, takes virtually no time to do that extra step.
This is my bias: I am very partial to a pocket clip and a relatively slim and lightweight light, since I don't like holsters and don't like to dig for 2+ seconds when I need light in less than 1s. If you want bright (or very dim or in between) light FAST this is it. It is quite uncomfortable to look directly at the diffuser cap with this light on max!
my 2c
If you want something smaller the Wee and Raw lights by Lummi are great, but neither have a pocket clip and battery life suffers.