A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph. Typically a light meter will include a computer, either digital or analogue, which allows the photographer to determine which shutter speed and f-number should be selected for an optimum exposure, given a certain lighting situation and film speed.
Light meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene. They are used in the general field of lighting, where they can help to reduce the amount of waste light used in the home, light pollution outdoors, and plant growing to ensure proper light levels.



read the rest here: http://en.wikipedia.org/wiki/Light_meter

I've had a Minolta Autometer IVF (I think that's the full name) for 7 or 8 years. Don't much use it anymore to be honest as I just use my camera.

I will say a light meter helps you learn exposure, and a good meter is handy for studio work, but I tend to just go by histograms now when shooting.

I don't know what all you shoot, but a light meter isn't something most people ever bothered using, and certainly probably even fewer use one now.

That's fine if you're shooting digital, but what about film?

I'm kind of looking for a cheap, small light meter to use with my medium format folders. Currently, I'm just guessing based on sunny f/16 (not sure how that's been working out, as I haven't processed the film yet), but I'd like something a little more accurate, especially for shady or indoor shots. Mostly I've been shooting with the folders while out hiking, so I don't really want to lug my dRebel along all the time, just to use it as a meter.

I DO have a Weston Ranger 9 light meter, but its not accurate, since the type of battery it uses is no longer made.