This is a non-exhaustive list of game servers that I commonly join.
The servers are ordered by which game they're running.
Razorback - Phoebus Apollo (razorback95.com:28503, what a certain Discord uses to host their dumb events. Other times a ghost town. Don't bother.)
FFA @ qw.servegame.org (qw.servegame.org:27500, populated everyday - asskicking courtesy of pros guaranteed!)
TastySpleen DM (tastyspleen.net:27916, the most popular Q2 server!)
MooTown Chaos Revived (mootown.mooo.com:27910, populated every Sat & Sun at 12:00 PM Pacific, every Sat & Sun at 5:00 PM CA time, according to it's owner Trixie.)
Retro FFA (ffa.q3retro.com:27960, commonly populated Q3A server!)
Q4 Funhouse (68.232.174.28:28004, where I met a former friend of mine.)
Zora's KeyCoop (zzora.freeddns.org:25555, runs on 225f - but can run on 224v to any 227 version from a to j, and possibly k as well!)
Sniper's Paradise (172.117.35.81:7780, "Who Loves You Baby!")
Any Gibblets.com server (IP/hostname and port may vary, admin is New Zealand player no0ne.)
Barbie's Monster Hunt (barbies.world:7777, mostly populated MH server.)
IMPORTANT: In order to play UT2003 online today, and see servers in the in-game browser, switch the master server to OpenSpy. Same thing applies to UT2004.
Teutonica (88.134.119.161:7777, EU server, modded to hell, occasionally gets populated.)
IMPORTANT: Same thing as UT2003, switch to OpenSpy in order to play online.
NOTE: Most servers on this section run the 3SPN (TAM/Freon) mod, which has a competitive scene, btw.
Slice of Life (SoL) Noob-Friendly Freon (72.5.102.229:7777, a noob-friendly server, staying true to it's hostname.)
House of Carpe (HoC) Public Freon (74.91.115.167:7777, another popular Freon server.)
{New}International TAM/Freon (31.186.251.103:7777, yet another popular TAM/Freon server.)
The Sandwich Substation (66.85.14.240:27666, mostly populated during evening to midnight)
Blue Monkey AWE (74.91.127.183:28961, the most popular CoD1 server today.)
{RA}The Real One (74.91.125.204:28960, commonly populated CTF server, where Suda recorded the COD1 segment of his first "I played every Call of Duty" video, and where he first met ImBroly.)
[OUTLAW]eXtreme+ 3.0 (64.94.238.161:28960, constantly populated.)
IAF Playground (108.61.100.116:28960, populated around evening to midnight.)
NOTE: Mod may vary from server to server. One may run Jaymod, the other may run silEnT for example. The servers listed here will have their mods after the IP and flavor text.
SIDENOTE: The grand majority of ET servers save your XP using some kind of database. This is generally reflected in their hostnames, which may include 'XP save' in some form.
F|A Recruiting (108.61.18.110:27960, the most popular WolfET server today.) - Jaymod
Original Maps USA (Bea, Pika & Co.) (216.155.140.212:27960, not that populated most of the time, but still good to include nonetheless.) - silEnT
The Old Farts SilentMod (68.232.172.16:27960, another uncommonly populated server.) - silEnT
Los Magnificos Silent XPS (108.61.120.56:27960, sometimes populated latin american server.) - silEnT (chosen by them for it's anti-cheat, according to a Discord call with them.)
More servers to come.