Opposing Force
Often defined as the best Half-Life 1 expansion. It involves you playing as the "opposing force", more commonly known as the HECU, as a soldier named Adrian Shephard. The expansion adds several new weapons as well as new enemies alongside the ability to lead squads of soldiers. The expansion has a few events that crossover with the original Half-Life, however for the most part Gordon isn't seen. After the game, Adrian is placed into stasis by the G-Man, who forgets where he put Adrian and as such Valve will not make another game with Adrian Shephard.
Half-Life Opposing Force, a great expansion but a shame that it's titular character was forgotten forever.