USS Darkstar takes place in 2066 on board a Zoological Research Vessel in which they study and investigate various aliens from unexplored sectors of the galaxy. You are Dr. Gordon Freeman, a brilliant researcher who has always had problems waking up on time...
It has a great intro, you encounter many "cut" creatures of HL (such as the kingpin, mr. friendly), there is a great atmosphear, a good gameplay, some incredible sequences such as one where an Ichtyosaur is in an aquarium which breaks and floods the level, really Jaws-like, many fights...
It is from the same developer of They Hunger, if you know it. USS Darkstar doesn't have really any flaws : not too short/not too long, no impossible fights, some challenges here and then, diversity in sequences... Well, that's it.