If you're talking about a tiny Altoids-type PSK, a small coin cell light including the $1.00 Countycomm ones is about all you can fit. Photons look nicer and some models have more features, but really they all do the same thing. And the Arc AAA is a wonderful light, but for the price of one of them you can buy a bag of 20 Countycomms. The CC lights are among the greatest of nerd bargains. I buy them in bags of 20 and give them out as presents all the time. They are always appreciated. Carrying several is good too, for redundancy or in case someone else needs one. Their main drawback is they are not very waterproof, but neither are Photons.

Another nice light is the UKE 2AAA eLED, $11.95 from www.brightguy.com. It's bigger than the Arc, but costs less, is waterproof to 1000 feet, and runs for 20 hours on two AAA's (regulated with boost circuit). It comes with a hat clip which is nice for handsfree use if you have a baseball cap. I think this is a really good PSK light if you have the space for it.