These are my favorite lights for the money. The batteries will work with your Photon too, plus if your photon died because the LED quit, you can swap the LEDs from this light and revive your photon.
For something of reasonable cost, I really like the Fenix LOP SE. The lower powered version. Three output levels... A nice low, a good medium and a high that is very usable, but certainly less bright than the Cree version.
For a long regulated runtime, usable output, and very reasonable cost it's tough to beat the Fenix E0. Some others are brighter and some have a better finish, but for the features that really count this is a nice light for the keychain or pocket.
I like my Fenix E1. Wouldn't mind an E0 either. I also like my Streamlight Keymate. It uses 4 button LR44 batteries, which are cheap if you get them online, but most people don't like lights that use button batteries (except for the small cheap ones like ones carbine15 linked to since those are so cheap, it doesn't matter if they're button battery lights). I like those cheap button ones too, but I prefer the overall package of the Keymate and E1 (and E0).
My keyring sports a Peak 3-LED , brass ,lug bodied Matterhorn and a photon freedom. The freedom has the SOS mode should I ever need it. The heft of the Peak makes an ideal fob for me and being a light it has a usefulness as well. I am a Peak freak. The Caribbean is among my favorite lights.
SL Keymate and Fenix E0 are great keychain lights for my uses. Both are reliable so far, although I haven't had them long enough to vouch for their long-term reliability.
I also always carry a Mag Solitaire that is still going strong after 16 years of keychain duty. That's reliable.
The E0 is twice the price of the Keymate and is a little bigger. If I had it to do over, I might just buy a second Keymate with plenty of extra button cells rather than an E0.
I actually have three of the Dealextreme lights on my keychain, all running higher-efficiency LEDs from the Jeled eBay store (such as these). I have one clear DX light with a white LED sanded down flat (pure flood), another (also clear) focused, and for my third I bought some 2032 coin cell batteries from DX and swapped in a Jeled red LED, and put that into one of the black DX keychain lights. My original intention was that those would be good for night vision, but they actually too broad spectrum for that (looking at them with a DVD, I can see a significant amount of yellow and even a tiny bit of green in there) -- though they are much less disturbing to others than the high intensity white ones, and for lighting up a pathway, won't impact night vision nearly as much as the white (even though they wouldn't work for say reading up close and keeping night vision).
I also have a Fenix L0D CE on the way. 4 Lights on one keychain, all ridiculously bright.