Star wars Battlefront II (2005) still has a lot of players, but the best servers generally have discord based communities
iirc there are still people playing Halo, but I reckon it is pretty rare to find a full game on PC. now if you were willing to drink poison and buy an xb1 you can get the masterchief collection which DOES have people playing halo, but i would not recommend that honestly. I got the collection mostly for the single player, and i was able to find a few games of classic halo ce, I am not sure how many people actually play it consistently.
Halo 2 online is now free to play. It's abandonware now, so it's neat. https://www.cartographer.online/
Eldorado: Halo Online is still active with about 50-100 players last time I checked (1 week ago). You will only find custom games however, but that's where the fun is anyways. The installation process is difficult however due to Microsoft claiming ownership of the halo assets and discouraging download links.
Not exactly. They're both by two different developers, with UT having been published and developed by Epic Games on one of the first versions of the Unreal Engine while Q3A was developed by id Software on the Id Tech 3 Engine. Not to mention the fact that while Q3A has a mashup of different characters some of which are from other games by Id (Such as Doom or previous Quake Titles) Unreal Tournament has its own roster of different teams and characters from the same fictional universe. Not to mention the fact that UT99 had weapons with alternate fire functions that drastically differed from the weapons you get in Quake III Arena. I apologize for the tangent but anyway...I know for one thing that the Half-Life community is alive especially in terms of modding. One of the longest-running mods that comes to my mind is Sven-Coop which is almost twenty years-old at this point. Besides that I know that the developers of the old Half-Life mods Poke646 and Poke646: Vendetta released an anniversary pack for both mods on moddb and back in August someone released a very well-done mod called Half-Life: Echoes there too. There are a couple more singleplayer mods I can recommend as well as some Multiplayer mods, some of which are actually available for download on Steam but unfortunately there usually isn't a lot activity on their servers. Unreal Tournament also has a thriving mod community though from what I've seen most of the content is usually mutations which alter gameplay or a custom levels/maps. It's sequel Unreal Tournament 2003/2004 also has a somewhat active modding community though I personally think it's being overshadowed by developers that make mods for Unreal Tournament 3. As for Quake, Quake II, and/or Quake III...I actually haven't checked out their modding communities all that much to be honest. I do know that a lot of people have been porting custom maps from Q3 to use in Quake Live but I'm not exactly sure about the scale of it.