Alex Rodriguez hit the first pitch he saw in the first inning over the leftfield fence at Yankee Stadium, a 3-run shot. His 500th homerun, like his first and 499th, came at the expense of the Kansas City Royals.
UPDATE: In a game I didn't watch. Barry Bonds hit a homerun in the second inning against the San Diego Padres, the 755th of his career, tying Hank Aaron's all-time record. One never knows, but I'm more inclined to believe Bonds used performance enhancers than I am Alex Rodriguez, so there's no joy in the latter achievement for me.