Bungie specifically allowed this to happen with the old Myth franchise. They open-sourced their bungie net code when they shut it down. Since then, several fan sites have kept the servers alive. The game was published in 1997.
I believe it was actually the work of a member of the Halo PC community; one non-affiliated programmer aliased as "btcc22" who kept it alive.
Soon after the GameSpy services shut down, he wrote an emulator to replace the official master server. Bungie then (shockingly) released v1.10 to point the game to the new server, and stepped in and ate the cost of the AWS services used to power the lobby.
Source: I gave my life to that game for a very, very long time and still check in on the community every now and then.
Fun fact: Eric Koger, the CEO of ModCloth.com, also spent quite a bit of time reloading his pistol and rifle.
Bungie released the original binaries and assets for all three games (http://trilogyrelease.bungie.org/). You can play these on vintage hardware, Aleph One just lets you play on modern hardware.
While I say "original", these are different from the retail versions of the original games, which required serial numbers. Bungie's Jason Jones removed the serial number check himself, without the original source.
