The Boston Globe ran an article today, "Modders" breathe new life into video games, which talks about game modifications. Of course, Counter-Strike is mentioned: [Thanks Fragmaster @ PHL for the news]
"Counter-Strike used the same underlying engine as Half-Life, but the mod had completely new maps, weaponry, graphics and sound effects."
"Half-Life's owners liked it so much, they decided a year later to release Counter-Strike commercially. They wound end up selling 1.5 million copies, generating $40 million in sales. The Internet game remains among the most popular around."