Battle.net
Battle.net launched on December 31, 1996, alongside Diablo — the first online gaming service built directly into the games themselves rather than requiring a separate client. Players could connect for free, chat with other gamers, and join multiplayer matches without subscription fees, a model that stood in stark contrast to the paid services of the era.
The service grew into a massive social platform where players across Diablo, StarCraft, Warcraft II, and eventually World of Warcraft could communicate and compete. Ladder ranking systems, achievement tracking, and persistent profiles gave players reasons to keep logging in.
From around early 1999 (I guess) until I deparated Blizzard in early 2000 I was, in addition to VP R&D, the battle.net lead, or rather, the only person working on battle.net. Yes, we didn't spend enough money on it!
Battle.net needed a lot of love because it was bursting at the seams. At least one of the servers would crash every day. There was no monitoring. No one knew how many players were using it short of connecting to every server and checking the count.
Even so, the architecture of the service was brilliant and I learned about building online services, which was of great utility in building Guild Wars online services.