He made his permanent name with the much-revised-and-appended "Seven Words You Can't Say on Television" originally released on his 1972 "Class Clown" album. When Pacifica Radio WBAI-FM played "Seven Words," it sparked an FCC sanction which eventually came before the Supreme Court (ruling: indecent but not obscene), giving Carlin his place in the free-speech history books.
(Ironies abound: Most of the words on Carlin's original list have made it to TV, with the qualified exception of what newscasters now delicately refer to as "the F-word." And it was the one word of the seven that he always felt was unfairly grouped with the rest--"a friendly word!"--that indirectly brought the FCC back into the indecency game thirty years later.)
Here are two tracks--"Drugs" and "Birth Control"--from his "FM & AM" album released earlier the same year.
The content's a little dated now, and they don't have as much of the manic intensity of his later stuff, but this is where it all started: The voice. The cerebral word play. The distrust of the establishment.
By the way, now that he's gone, you can expect a lot of inspirational "forward this to all your friends" emails with writings attributed to Carlin--none of which he wrote--to start reappearing in your mailbox:
Illegal immigrants and the gas crisis
Being a "bad American"
New rules for 2006
The paradox of our time
Some of them--especially "Paradox"--have moments where they come close to reproducing his voice, but all fall short of the genuine article. Out of respect for Carlin, delete them when you get them. Don't forward them.