National Hockey League
Jim Riley is the only person to play in the NHL and MLB, but several other NHL players have been minor league players (e.g. Maurice Richard, Doug Harvey), national team players (ex. Jack McCartan, Gordie Clark), minor league owners (Mario Lemieux) or had other roles in baseball.
Major league ballparks have sometimes been converted into hockey rinks to host outdoor NHL games, especially at the time of the annual "Winter Classic" traditionally played on January 1st. Fenway Park, Wrigley Field, Nationals Park, Busch Stadium and Citi Field are among the ballparks that have been converted into temporary NHL venues over the years.