Robin Williams was a big fan of the series, and was actually in talks to do a voice for the games. Scheduling conflicts prevented this.
If you reverse the wails from the head crab victims, you'll hear them screaming "help me," indicating that they are still aware of their surroundings and the extreme pain that they're in.
When you first meet Barney, he says, "Now, about that beer I owe you". This is a reference to the original Half-Life. Before the disaster, each of the characters had a set of "I'm busy" lines to brush you off if you tried to talk to them. One of Barney's was "Hey, catch me later and I'll buy you a beer."
It is the highest rated PC game of all time.
The original opening sequence of the game placed the player on a hill near a Utopian modern-day city. The sky was blue and the sun was shining. Gradually, as the G-Man spoke, the sky turned red and darkened, and the city turned black. In a flash of lightning, the Citadel appeared in the middle of the city. A train pulled up and the player climbed on and, after an extended opening train reminiscent of the original Half-Life (1998), arrived in City 17.