Half Life: Blue Shift is the second stand-alone expansion pack for Half-Life and was released in 2001. Returning to the Black Mesa Reasearch Facility as a security guard who allied with Gordon Freeman in the original Half Life, players explore previously restricted game's levels, fight hostile creatures or characters and solve a variety of puzzles to advance.

Opposing Force returns to the same setting as Half-Life, but instead portrays the events from the perspective of a U.S. Marine, one of the enemy characters in the original game. The player character, Adrian Shephard, is sent in to neutralize the Black Mesa Research Facility after a scientific mishap causes it to be invaded by aliens, but quickly finds that …

Originally, Gunman Chronicles was developed by Rewolf Software as a mod for the first person shooter Half Life; after negotiations Sierra released it as a retail version in 2000. In Gunman Chronicles, the general of the Gunmen fell in battle with the Xenomes, a strange alien species which suddenly appeared on the planet Banzure Prime. Now, a mysterious presence has …