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.

Opposing Force by Ol'No.16 September 11, 2021

Opposing Force 

A Half-Life re-release in which you play as one of the Soldiers sent out to kill Gordon Freeman.

"Have you played Opposing Force yet?"
"No, how is it?"
"Better than the original."

Opposing Force by Stankfoot March 2, 2008