The "sequel" to Counter Strike Source came with the creaton of the all-mighty Source engine and was developed by Turtle Rock Studios. It was released in November 1, 2004 (2004-11-01) on Windows and on June 23, 2010 on the Mac.
Following on the footstepps of it's predecesor it quickly created a community and became the
#1 Online Action Game in the World!!!!!!!!!!!!!!!!!!!!!
Counter Strike:Source is not really a sequel to Counter Strike but it's more like a remake. It had the same guns, and mostly the same map rules, with the strongest point being on realism.
Being created with the Source engine however brought a some changes to the game, the most noticeable is the graphics and the game physics, thus Counter Strike came even closer to achieving it's goal realism.
By using the Source engine CSS also benificiated of SDK(Source Developement Kit), a tool modders used to the maximum.
The release of CSS also came with critiques, most of them being about the fact that it had the exact same weapons(big downer for me), and a more innacurate recoil, reason why the Counter Strike 1.6 community is still alive and kicking today.
Also unlike 1.6 Valve din released several big updates that was recieved pozitively by the community.
The updates improved the on screen radar, scoreboard, added a killcam, achievements and much much more.
To understand more about the game please see this videos:
The power of SDK
Next on the list is the Team Fortress both games in one post then we go to COD and MOH and some UT-engine games.