Counter-Strike 1.6 was modification to Half-Life. It has newer versions like Counter-Strike: Source or Counter-Strike: Global Offensive. They used GoldSource engine to create (everyone's favorite) game of cops and thieves. Steal all hostages (
cs_*) and place and detonate a bomb (
de_*) became favorite modes. Why? They are simple to understand but you can get better and better by practice and cooperation. How? For example by your choice of weapons. Do not have Internet? You can play it against bots. Another aspect of the game was modding and custom maps which let the community grow and keep the game alive even more then 15 years after its release (which is a lot compared to new games which have problems to keep their community for 1 year).