From "40th anniversary: Stargell hits one out of Dodger Stadium" by Chris Jaffe at The Hardball Times:
Forty years ago today, Willie Stargell did something that no one else had ever done before and no one else had ever done since. He smacked a ball completely out of Dodger Stadium. Not just out of the playing field, but out into the parking lot itself.
On May 8, 1973, the Dodgers hosted Stargell’s Pirates in a night game at Chavez Ravine. In the top of the fourth inning, Stargell came up with a runner on against Dodger hurler Andy Messersmith when he hit his historic moonshot.[...]
Oh, I should note that today isn’t the 40th anniversary of the first time a ball left Dodger Stadium. Wait, what? Yeah, you see, while Stargell is the only person to do it, that doesn't mean he did it only once. Stargell first accomplished the feat in 1969, and today is the 40th anniversary of his second parking-lot shot. We’re still waiting for a third occasion.
UPDATE: Chris chimes in with a correction:
Apparently Piazza did - sort of. He hit one that hit the roof and bounced out. So did McGwire, apparently. A commenter noted this at THT.