Carlin, an irreverent stand-up comedian, actor and author, died of heart failure at age 71 in California on Sunday.
"George Carlin was the living embodiment of the First Amendment. In the traditions of Mark Twain and Jonathan Swift, he dealt with the insanity of the world with the one-two punch of humor and honesty with no apologies," Williams told E! Online.
"He was one of the greats," Stiller said in a statement to the entertainment news Web site. "And he will be missed. (He) was a hugely influential force in stand-up comedy. He had an amazing mind, and his humor was brave and always challenging us to look at ourselves and question our belief systems, while being incredibly entertaining."
"Dammit," Smith, who directed Carlin in several films, wrote on his Web site. "I was happy to have met the man, let alone worked with him."
Leno described Carlin as a voice of his generation, adding: "His comedy took on privilege and elitism, even railing against the game of golf. He never lost that fire. May he continue to inspire young people never to accept the status quo."