Built MaNGOS in 2004/2005 with a bunch of other people as the first Open Source World of Warcaft server. Over the years it was the biggest project on Github for a while, even trumping projects like Ruby on Rails in forks and at some point most if not all private servers were using it.
Eventually Nostalrius forked it, slapped marketing and a website on it, became huge and those guys got to visit Blizzard Entertainment, in the end leading to Blizzard finally giving in an providing World of Warcaft Classic as a service.
Cost me tons of money to keep the project running because we were constantly under attacks from god knows who and in the end burned me out after a decade of doing this.
To this day private servers are using it, and probably six figures of people play on those.