This decade old game is still one of the best out there!
You play as a physicist, Dr. Gordon Freeman, who is involved in an accident with an alien artifact causing many different hostile aliens to invade the facility where he and hundreds of other scientists work. You must fight your way to find the other scientists avoiding being attacked by powerful aliens and U.S. Military personnel who have been ordered to purge the facility of all life, humans included.
The game includes nonstop action and a great storyline. It is definitely one of the best games out there plus this version of the game runs on the, easy-to-use, Source game engine. It certainly won't disappoint.