Kansas City Royals at Chicago White Sox

The White Sox dressed in 1983 uniforms on the 25th anniversary of that team, and faced the Kansas City Royals who did not wear throwbacks.

Cleveland Indians at Seattle Mariners

Depending on who you ask this game was a flashback to either 1987, 88 or 89, featuring the visiting Cleveland Indians and the home team, the Seattle Mariners.

Pittsburgh Pirates at Tampa Bay Rays

On African-America Heritage Day, the Pirates dressed as the Pittsburgh Crawfords and the Rays dressed as the Jacksonville Red Caps for their interleague game in St. Petersburg.

Seattle Mariners at San Diego Padres

San Francisco Giants at Kansas City Royals

Houston Astros at Tampa Bay Rays

The Rays dressed as the St. Petersburg Pelicans of the short-lived Senior Professional Baseball Association of 1989, while the Astros wore their "tequila sunrise" uniforms of the 1980s.

San Diego Padres at Cleveland Indians

Both teams wore uniforms from the 1978 season.

Oakland Athletics at San Francisco Giants

Atlanta Braves at Chicago Cubs

Thanks to Mark in Shiga who in the comments mentions this one, the Atlanta Braves and Chicago Cubs played a game to commemorate the 60th anniversary of WGN Cubs telecasts by both wearing 1948 duds. The Braves were the Boston Braves and the according to Bill Henderson, the game was "broadcast was in black and white with 1948-era camera angles (no centerfield batter shots, no instant replay)."

Chicago White Sox at Los Angeles Angels

The Angels dressed in their 1971 uniforms, in a game against the non-participating Chicago White Sox, who wore their black alternate jerseys.

Texas Rangers at Oakland Rangers

Celebrating the 40th anniversary of their move to Oakland, the Athletics celebrated with 1986 unis, while the visiting Texas Rangers dressed as their former selves, the 1968 Washington Senators.