Houston Astros at San Diego Padres

The Astros and Padres played a game in San Diego wearing 1976 uniforms.

Special thanks for this screenshot of Adrian Gonzalez to a guy who went by the name 8/3/2006 Astros @ Padres in the comments:

Detroit Tigers at Seattle Mariners

The Mariners dressed as the 1969 Seattle Pilots (who would later become the Brewers) and the Tigers wore their own jerseys from 1969.

Milwaukee Brewers at Kansas City Royals

Atlanta Braves at Tampa Bay Devil Rays

The Braves and Devil Rays played a game at Tropicana Field dressed in 1975 uniforms, the Braves, their '75 road unis, and the Devil Rays were wearing uniforms of the 1975 Tampa Tarpons, a FSL minor league team.

Washington Nationals at Milwaukee Brewers

Pittsburgh Pirates at Cleveland Indians

Chicago Cubs at San Diego Padres

Special thanks to Mark in Shiga in the comments who brings us this game where the Padres dressed as the 1948 PCL Padres.