Otherwise known as a Gaseous Tritium Light Source, a traser is a glass tube that has been internally coated with a phosphor - often green - and filled with tritium gas. As the tritium radioactively decays, the beta particles that are emitted excite the phosphor, causing it to emit visible light.
Trasers can last upwards of 10 years, and will continually emit light with no maintenance whatsoever. They are commonly found in expensive watches, military gunsights, and key fobs. Sadly, the key fobs are illegal to sell in the United States - mine was smuggled here by a friend of mine who visited the UK.
Traser is also the name of a company that manufactures and distributes these products. Many of their products, however, are only available in Britain due to the aforementioned legal unpleasantries. | 
