Counter-Strike is a no.1 multiplayer game, played in many world cyber tournaments.

History:

On 24 March 1999 Planet Half-Life opened its Counter-Strike section. Within two weeks, the site had received 10,000 hits. On June 18, 1999, the first public beta of Counter-Strike was released, followed by numerous further "beta" releases. Counter-Strike 1.0 was released around Christmas 2000. On April 12, 2000, Valve announced that the Counter-Strike developers and Valve had teamed up.

Version history

1. Beta 1.0 – 19 June 1999
2. Beta 1.1 – 27 June 1999
3. Beta 1.2 – 20 July 1999
4. Beta 2.0 – 13 August 1999
5. Beta 2.1 – 17 August 1999
6. Beta 3.0 – 14 September 1999
7. Beta 3.1 – 16 September 1999
8. Beta 4.0 – 5 November 1999
9. Beta 4.1 – 1 December 1999
10. Beta 5.0 – 23 December 1999
11. Beta 5.2 – 10 January 2000
12. Beta 6.0 – 10 March 2000
13. Beta 6.1 – 24 March 2000
14. Beta 6.2 – 26 March 2000
15. Beta 6.5 – 5 June 2000
16. Beta 6.6 – 22 June 2000
17. Beta 7.0 – 26 August 2000
18. Beta 7.1 – 13 September 2000
19. 1.0 – 8 November 2000
20. 1.1 – 10 March 2001
21. 1.2 – 12 July 2001
22. 1.3 – 19 September 2001
23. 1.4 – 24 April 2002
24. 1.5 – 12 June 2002
25. 1.6 – 15 September 2003 (released with Steam)

(wikipedia)

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Counter-strike 1.0

CS 1.0 was different than nowaday CS. The models were different, maps were different, weapon type and strenght were different. You can see some of CS 1.0 here:

Counter-strike 1.1 - 1.5

From my experience, the versions 1.1 - 1.5 didn't differ very much, there were just bug fixes, and other minor changes.

Counter-strike 1.6

Released in 2003 with Steam, it was different from previous versions not to mention new Steam system, which caused the birth of "non-steam" community. The game had changed quite a bit with the new weapons and stuff, but the game engine remained the same. Now, it is most popular version among the players.

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Counter-strike Condition Zero

Condition Zero is another version of CS, which contains a single player mission pack with the player unlocking maps and more efficient bots as they pass certain requirements for each map while playing as a counter terrorist. It also has the multiplayer option. The game consists of updated model, textures, maps, but game engine still is from Half-Life.

Personally, I've completed the single player a couple of times and I think it's quite okay :)

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Counter-strike Source

Finally, the game was released on a new engine (Half-Life 2). Whole graphics, models, and even gameplay has changed. Some of CS maniacs and experts still prefer CS 1.6.