Examples of Computer-Found Optimal Strategies in Games
The best known example is probably checkers (also known as draughts), which has been solved recently in 2007 (the game is a draw). Other examples are listed in the Wikipedia page on solved games; notable among them are connect four and nine men's morris. Additionally, several chess endgames have been solved.

This perhaps doesn't seem like an answer to your question, but if an expert (such as Marion Tinsley) loses to a computer program, then the computer must have found a "more optimal" move.