I played the original Natural Selection mod of Half-Life back when I was around 10 years old and have loved it ever since. I love what they've done with the sequel and it's definitely my #1 game. Why? It has great Alien vs. Marine 1st-person gameplay unlike anything I've seen, and it has a fantastic RTS element where one player on each team has to be a commander and is basically playing Starcraft with his real-life players. Highly recommended.