Neowolf said:
Yeah, there have been 4, starting back in the 40's up to the present day.
Alan Scott
Hal Jordan
John Stewart (the one from the JL cartoon)
Kyle Raynor (the current GL in the comics)
I'm not certain that's the right order, and as someone else pointed out, the Lanterns are a sort of intergalactic police force, not just one person.
In case you were wondering, other DC heroes who have had multiple identities are The Flash (3 that I know of), Robin (at least 3, possibly 4), and Green Arrow (2 I think). There are probably others, but that's off the top of my head. 
FYI, my Avatar over there to the left is the Kyle Raynor GL.
Guy Gardener was also a Green Lantern. He later got Sinistro's yellow ring and fought crime with that for awhile before discovering that he had some sort of alien heritage. He then drank some kind of (magic or alien) potion in South America and gained the ability to grow armor and weapons on himself and changed his name to Warrior. During this time, the series (WARRIOR) had some knockout artwork by Mitch Byrd.
As to other heroes...well, that gets confusing if you count all the future/past and alternate universe versions. For example, there were 2 Earth Lanterns before Allen Scott. One was in ancient China and another was Metroplis at the beginning of the 20th century. And there were a million Flashes.
Not counting guys like that, there were 3 Green Arrows: the Golden Age version (I think he may have been "Oliver Queen", as well), Oliver Queen, and his son (Connor Hawk?).
Robins? The Golden Age Dick Grayson (who died in the Crisis On Infinite Earths mini-series), modern Dick Grayson, Jason Todd, and, uh, the new kid (I don't read Batman). Plus the female Robin in Dark Knight Returns.
