off of the top of my head:
>Empires
old-ass Half Life 2 mod from 2008 that's still supported and regularly played to this day. it's an RTS/class-based multiplayer hybrid like Natural Selection except it's set in a sort of retrofuturistic race war with an emphasis on janky source vehicle combat. it probably sounds like a fucking mess from that description but it's actually really fun, and AFAIK it's steam version is technically a standalone title now and doesn't even need HL2 to run it
I haven't played it in a couple years myself since I became a nightshift wagie but it still gets 50~ players each Saturday and Sunday and a few of them have been playing since release

>NeoTokyo
another HL2 mod from 2009, it's not officially supported (I believe the source code has been lost so the game couldn't be handed off to anybody) but still gets 30~ players on Friday pickup games organised by a steam group. it has an amazing art direction and setting based off of Shirow Masamune and it's gameplay is styled off of Counter Strike except with cloaking nip cyborgs, but unlike actual Counter Strike it struck a good balance between chaotic fun and hardcore, predictable, skill-rewarding gameplay rather than going full-bore into the "hardcore" aspect; meaning it has fun stuff that spreadsheet-brained CS:GO-era autists would screech at such as large team sizes, physics props, friendly fire and so on. also has a standalone steam version.

>Quake III Arena
doesn't really need an introduction, it's Quake and I believe the most active Quake title to this day. I haven't actually played any games of it myself but I assume it's heavily gatekept by a small, elite group that have been playing since release 25 years ago and are able to do shit like accurately map long-range airshot trajectories in their head from any angle, and I don't mean this in a "heh heh are you up for the challenge buddy" kind of thing either. also, still sold at $14.99 US despite being a quarter-century old