Riverfront Stadium
Fenway Park
Old Yankee Stadium
Cleveland Municipal
Busch Stadium
Oakland Coliseum
Candlestick Park
Dodgers Stadium
Jack Murphy Stadium
Shea Stadium
New Yankee Stadium
Wrigley Field
US Cellular Field
Petco Park
AT&T Park
Nationals Stadium
Angels Stadium
Kauffman Stadium

I always thought I'd been to more parks until I did this list, but I have seen every team in the league at least once.