Links

The Game
Counter Strike Source - Latest version with better graphics.
Counter Strike: Condition Zero - Updated version of the below with a single player mode.
Counter Strike 1.6 - The original Counter Strike
Counter Strike 1.6 (FREE) - If you own Half Life (original) you can get Counter Strike for free!

Counter Strike Mods
GunGame
GunGame is a style of gameplay originally made popular by cagemonkey. The objective is to get a kill with every weapon in the game in order, starting with the measly Glock, and working your way up through the shotguns, assault rifles, etcetera, until you reach the final level: the knife.

Wikipedia
Counter Strike on Wikipedia. If your after a good history of the game, Wikipedia has an entry all about it.

Conter Strike Online, is a mod of Counter Strike Condition Zero set for the Asian market.