Of current Major League:

Fenway Park
Anaheim Stadium
Oakland Alameda County Coliseum
Camden Yards
Citifield
Yankee Stadium

Skydome (Toronto Blue Jays),

Dodgers Stadium

Petco Park

(Some have probably changed their name since then).

Also been to former Major League stadiums:
Jack Murphy Stadium
Mile High Stadium
Old Busch Stadium
Candlestick Park

And Minor League Parks:
The Diamond (Richmond Braves)
Harbor Park (Norfolk Tides)
Jackie Robinson Ballpark (Daytona Cubs)

Coca Cola Field (Buffalo Bisons)