The game has been available since the 29th of June 2000.
The expansion set called Lord of destruction arrived in 2001.
It's still one of the most played games and in 2001 it was the most popular game to play.
It is a very nice game in my own opinion and I've had lots of fun with it.
I'm definitely waiting for Diablo III now aswell

You can buy a version of Diablo II on the Blizzard Store, located here:
US VERSION:
http://us.blizzard.com/store/EU VERSION:
http://eu.blizzard.com/store/