Counter-Strike
Thank you, Counter-Strike, for changing my life. The gameplay was a very simple concept. You were either a terrorist (T) or a counter terrorist (CT) and had to kill the other team. As a CT, you would also have to free some hostages and lead them to safety or find and disarm a bomb within a given time frame. Therefore, you had two ways to win: kill everyone on the other team and complete the objective (as a CT) or make sure the other team doesn't complete the objective in the allotted time (as a T).
If you had a headset, you could chat without typing to the players in the game. There was a way to talk to just your team, to promote strategy. But, the best way to play was at a LAN party or in some sort of common area, this way you could trash talk outside of the game's chat capabilities. Seven years later, the game still looks pretty solid. The graphics were very good for it's time and Counter-Strike was the reason to buy a new video card.
The buildings were all very well done, as were most of the environments. While you didn't have many options as to what character mapping to use, they all looked pretty solid as well. As it was a modification to an existing game, some of the independent maps weren't as nice looking, but at least people were trying.
The sound was an important factor to the game. If you found a good hiding spot, you could turn up your speakers and listen for someone's footsteps. Sometimes, they were too careful and walked slowly. That way, you couldn't hear them and they could sneak up on you. Otherwise, the game sounded very average. You had guns firing and explosions. It's kind of hard to make them sound bad.
I believe Counter-Strike is one of the most influential games of all time. It became the reason to get high speed internet, to purchase a better video card, to ingest high amounts of caffeine and stay up all night, to go to LAN parties. Its appeal destroyed the boundaries caused by genres, as fans of RPGs, sporting games, and everything else played this wonderful title.
Counter-Strike is still very popular, with three times as many servers online as the second most popular FPS, showing its staying power. In recent times, the sequel has been released and expands on CS in every way, but the original is the title that changed the world.
