Bore lights are available through a number of vendors. As of Dec 2005, SocalGuns had them for $6 in-store, and I understand that Wal-Mart sells adapters that turn practically any flashlight into a bore light. The point of this is that bore lights are inexpensive and readily available, so everyone should own one.
In form, a bore light typically resembles a pencil with a thin, angled piece of clear polycarbonate sticking out the end. The purpose of this angled polycarbonate is to provide a channel for the light to get into the actual barrel of the firearm while you look down the other end. This requires opening the action of the weapon and keeping it open.
Naturally, anyone who performs this procedure without first clearing the magazine is a Darwin Award candidate. It is pretty much impossible, however, to perform this procedure without having first cleared the barrel of any live round.
A typical, pencil style, battery operated bore light.:
An LED in a shell casing transmits light down the bore for easy inspection.:
The orange piece reflects ambient light into the chamber. Requires no batteries and can act as a flag safety when inserted into the action of your firearm.: