Go to the Enter Cheat Code screen and use the following codes to unlock cheats and extras. Some passwords were provided by the PS3 Major League Baseball 2K7 board.

Password - Effect

themick - Adds Mickey Mantle (Rating 100) to Free Agents.
Black Sox - Unlock all cheats, with the exception of Topps cheats.
Game On - Unlock all extras.
Derek Jeter - Unlock everything but the Topps baseball card cheats in My Cheats.
mightymick - Unlock the Mighty Mick Topps cheat.
triplecrown - Unlock the Triple Crown Topps cheat.
m4murder - Big Blast.