I think I have already mentioned that I like Heroes of Might and Magic series (and King's Bounty). In that world, HoMM3 is the masterpiece. Game and its official expansions were developed between 1999 and 2000.
The first unofficial expansions soon came out...already in 2004 there was a full featured "hack" of the game where a Russian team injected a script language into the game files, and with that scripts you could control and extend the game. This made a boom in player base which drew more HoMM3 players to WoG (Wrath of Gods) version than to official sequels of which the successor, HoMM4, is absolutely haram. 5 was better but 9/10 Heroes players was playing WoG, and so on.
Now well into the 2010s, someone makes a static unofficial expansion called Horn Of The Abyss, but it has modern internet multiplayer built in. Another bunch of people remade the engine and created a HD launcher that, apart from fixing the obsolete graphics ratio, also removes any glitches that were present with the interaction of old graphics engine with modern OS. It works out of the box.
In 1999 I've played hot seat with my bunch of school buddies on old tube screen. HoMM3, in its graphical essence, is a late DOS game style. Couple of days ago I've played against an unknown human foe somewhere on the globe in HD resolution. I find that fascinating.
So now I can true multiplayer classic Heroes 3 in HD, or I can play WoG in HD. There is also 3rd unofficial option called VCMI, which is a complete open source engine clone. It is still not 100% compliant and can crash every now and then, but it can be compiled for anything - so now I can also play classic Heroes 3 on my cellphone.
This is my favourite game from late childhood/early adolescence period. You know that stuff stays for life. And now just by the power of its niche fan base, the game that has been officially abandoned and surpassed two years after being released, is alive, networked, and runnable anywhere. There were 1000 people active on the server at that moment days ago. And yeah the moment when I registered with user/pass/e-mail the modern way, in those old runic GUI elements of the game, was priceless.
There is official Steam HD version released few years ago. It sucks.