Source: CS Nation
I forgot the day CS was released but luckily CS-Nation pointed it out. I think a few more mods were released around this time too but my memory is not as good as it once was. :)

Counter-Strike turns five years old today! Released on June 18th, 1999, BETA 1 was one of the first major mods for Half-Life. From there, Counter-Strike blossomed into its current form. Even after years of content and gameplay updates, CS is still being updated rountinely with help from the controversial Steam platform.

Half a decade is quite a milestone and I must congratulate the CS team on such a grand accomplishment. Counter-Strike's success is unparalleled by any other online title. Everyone else on the Internet thinks that the CS community is just a bunch of immature kids. Well, if that's true, then I'm a damn pervert because I love you guys! Haha, not really. Get off my MSN list, you loser.

Anyway, yeah, five years!