counter strike source

I was on steam today you now the website that host game servers and also sells games well I came across a game called counter strike.I thought my self this game is old I use to play this when I was about 11 in 2004. so I downloaded it and play it , you would think a game so old wouldn't have anyone online but think again there thousands of people online from all over north america, south america, europe,asia, africa. what other games do you know that have lasted that look and people play and like it and it is being sold not given away for free