The Photon Micro Light is a highly useful and portable LED light, small enough that the whole thing can fit on top of a US dollar coin. The body of the light is made of a glass-filled plastic which is highly durable, and the light itself is very bright. The light has a hole in the back for a key ring, opposite the red, orange, white, green, turquoise, blue, or infared LED.
The original Photon, also known as the Photon I, has a simple switch action; just squeeze the body of the light to turn it on. There is no way to make it stay on, unfortunately. This feature came along in the Photon II, which added a small slide switch near the front to make it stay on. This is easily manipulated with a fingernail.
Among the Photon's colors, the turquoise is the brightest. In addition to its brightness, the turquoise model also has the special feature of having the LED leads gold plated for corrosion resistance. Do NOT pick up the turquoise model and shine it into your eyes just to see how bright it is - this will be painful. The blue is second brightest, according to my unscientific eyes. I haven't used the other colors, so I don't know how they work.
The Photon II uses one or two CR2032 lithium coin cell batteries. These batteries will be very unhappy if you let your Photon go through the wash (this is where those gold-plated leads on the turquoise model come in handy!)
An ultraviolet version of the Photon II now exists; this, unlike previous Photon models, does not require a small Phillips screwdriver to change the batteries.
The Photon I and II use a simple switch contact to turn the LED on and off. This is fine for most people, I suppose, but for those of us who have all sorts of geek toys, or just want the ability to change the brightness, microprocessor control is the way to go. Enter the Photon III, powered by two CR2016 button cells, controlled by a small microcontroller and two switches, and equipped with a water-resistant body. The Photon III has three levels of PWM dimming and a couple of strobe modes. Battery changes no longer require a screwdriver on the Photon III, as the battery cover snaps on and off.
Another microprocessor-controlled Photon creature is the Rav'n, which uses two CR2016 batteries, and is equipped with a tricolor RGB LED. The Photon Rav'n comes in two models, both intended to be swung or otherwise moved around to create a multicolored trail of light. One model creates a broken ribbon of colors, the other creates a continuous one.
If you want to always have a small, reliable flashlight with you that's small enough to fit on your keychain and bright as heck, get a Photon. They're a lot more durable than most other small LED lights, and are fairly inexpensive. The official homepage of the Photon Micro Light is at http://www.photonlight.com.