The most I hate about keychain lights is their short lifecycle. The Mini Tritium Glowring Keychain wins my heart by having the ability to glow continuously for 10 years!
It's perfect. Completely contained and self-powered, with a compact design and an ultra bright green glow (hello night clubs). And it's for less than $10!
According to wikipedia, Tritium (symbol T or 3H) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains no neutrons.
You need not worry about the dangers of the radio-active material, the particles emitted are so weak they don't even penetrate the skin.
If you were impressed by this keychain, check out this amazing light innovation that lasts 200 years (!), and without the use of radioactive material: