Titanfall 2 servers were brought back online after being down for awhile.

I still play Starcraft 2 occasionally.

Metriod Prime: Hunters.

It was the best FPS, arguably the only FPS, on the Nintendo DS. Nintendo has long since shut down their online service for the DS. However, if you go into your WiFi settings you can change your DNS to point to a server that spoofs Nintendo's credentials.

Thanks to this exploit you can play all the original DS games online with a legitimate game, on a legitimate console. There's even a discord for MP:H with a matchmaking channel, clans, and regular tournaments. (The same probably goes for Mario Kart DS)

Back 4 Blood was the game that served as the idea for this post.

I recently felt like picking up some cheap copies of it to play with a few friends, and decided to launch it once ahead of time just to test it out and see how it ran. I picked "Online" mode out of habit, feeling it would likely search for a bit before handing me 3 bots to play singleplayer. Instead, I actually got a decent group of people together several days in a row.

In B4B's case, while the developers visibly "abandoned" the game in news headlines, the form it exists in is very playable and generally bug-free, even if its ultra-highest-difficulty "endgame" allegedly lacks some refinement. It got a lot of outlash for not matching the playstyle of Left 4 Dead; having players use a deep system of roguelike-style upgrades. Since the enemies escalate in difficulty, those upgrades are often necessary and can connect with team strategy. It's now on PS+, and since it's crossplay, Steam players will get a lot of queue buddies. It's also playable with just 2 people since the other 2 characters will just be bots.

Quake 3 and Unreal tournament both released before the year 2000 and still have an active community

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Heroes of the Storm is still up after it stopped getting updates years ago.

My buddy still regularly plays EverQuest Online. These days, it’s sort of expected that you multibox and run an entire party, instead of just one character. He usually has his bots pulling mobs in the background of whatever other game we happen to be playing.

Age of empires 2. Still works.

I haven't played in a number of years, but I'm pretty sure fan-made servers are still running for Phantasy Star Online.

I was playing the GameCube version online well over a decade after that console was dead. Blue Burst on PC is well supported too, I believe.

Crazy now to imagine a console game letting you input a DNS server and IP address for online play. But it did.

I haven't played for years now but NeoTokyo still has an active community.

Post Scriptum (now known as Squad 44) is still alive. A very under-appreciated game and they are trying to revive it with a new dev team who have been doing wonders but the active player count is still well below 1k.

Ultima IV -

https://www.bestoldgames.net/ultima-4/play

EDIT: I realize I didn’t really interpret the question correctly on my initial read. This is meant more for old games, not sleeper online games that are just good in their own right without being live service. Perhaps a more fitting answer would be AssaultCube. One of the first multiplayer FPS games I played. There probably aren’t any official servers anymore, but community hosted ones were supported, so I’m sure it’s still around.

Straftat. Free to play, fast paced, 1v1 movement shooter. It’s a wildly under appreciated game that would hugely benefit from a small to medium sized, consistent player base. It does have a paid dlc, that mostly functions as a tip for the dev. The DLC has some cosmetics and a few maps, but it’s not really gatekeeping any of the fun of the game, plus it’s only $5 USD so I just bought it and considered it the price of the game.

Final Fantasy XI, the first final fantasy mmo still has a hard-core dedicated player base. It predates wow and launched on the PS2.

There's a number of mud games still around in their text glory. Love me a text mmo. Well most aren't massive. None are massive. Hence mud, multi user dungeon. But not massively multi user.

I currently play Starmourn which is very new by mud standards.

City of Heroes/Villains https://forums.homecomingservers.com/getting-started/

Guild Wars 1 - last month celebrated its 20-years anniversary :) I only started it in 2018, but it's a really solid game!

And both its developer and publisher are committed to keeping it alive as long as they can. It's been mostly automated in 2013, so apparently it costs very little to keep the servers running.

Call of Duty: United Offensive on PC. I just recently came back after a decade of not playing (and over 20 years after release) and the community is still there just smaller and the game is still just as fun

Istaria, previously known as Horizons. Still the only MMORPG (to my knowledge) with dragons as a playable race.

Rise of Nations was relatively active up until a few years ago. Unfortunately not natively supported on my device, haven't tested emulators.

A comprehensive list of verifiably playable games could be huge for us, patient gamers.

Subspace, also known as Continuum later in its life.

Team fortress 2 still has a massive community.

APB Reloaded. Just looked at ths 2 days ago and 200 people were playing. This was the GTA online before GTA online. They blew their load and spend the budget on the character creator and nothing else

I thought "that game had such a tumultuous development cycle and launch surly it's dead now right?" Wrong

City of Heros /villains is the one most shocking. Kept alive by private servers. God speed to that community

Stalker: Shadow of Chernobyl