Doom/Doom 2 is easily one of the best FPS games ever, it took the blueprints from earlier godfathers of the genre, including id's own Wolfenstein, put it in the Awesomewave oven for a year and a half, and spawned the one piece of shareware that every single computer had on it at least once in its lifetime.
I discovered ZDaemon in 2003-2004ish, and completely fell in love. It has your classic Deathmatch - Free For All BFG spamming goodness, and then it's got some modern game modes, which fit perfectly in the classic playing style. Team Deathmatch - simply put, pits red against blue in a match of who can destroy 100 of the other teams players with the Super Shotgun first? And it's fantastic. Capture The Flag is easily my favorite mode - it takes the lightspeed, fast paced nature of Doom's Deathmatch and turns it into a strategic, team-based, competitive game mode that's ready to steal hours of your life.
On top of the awesome game modes, the client is pretty sweet. It has a master server list that's populated with servers all over the world (plenty of which are in my lovely home state of South Jersey [yes, South Jersey is a different state than North Jersey, accept it] Loving that 5 ping!), with all kinds of game modes, including one I forgot to mention above, Co-Op! Seriously, having 8-10 people running around gunning down WALLS of Cacodemons and Imps is inexplicably satisfying. There is also a built in function called "GetWAD" which from what I've read was something programmed by someone from the community that automatically fetches the wads for you from the internet, kind of like how you download a map from the server in any modern FPS. There's a friends function, a built in IRC client, a built in tab for using the website, automatic demo recording, all kinds of nifty features.
The main thing that really astounds me is that the community is still going strong, and with plenty of people making new maps (myself included - I have an entire youtube channel on how to make maps for Doom), there's constantly new content. I checked the site recently and found out that there are some new game modes coming, Double Domination, and King of the Hill. Doom has proven to me in the 9 years that I've been playing online, let alone the FOREVER that I've been playing it since it came out, that it's invincible. It's one of those games that will always be played, always have something new coming to it (thanks for releasing the source code id - champ move), and will always have at the very least that last dedicated group of players.
If you like Doom, you need to check out ZDaemon, or quite frankly, any of the other numerous source ports (Boom, PrBoom, Skulltag/Zandronum, GZDoom, ZDoom, jDoom, etc.) and give them a whirl. Playing Brit10.wad Free For All with 12 people is insanity.
If you decide to play, add me to your friends list (Username: Jaundice), message me on IRC if you see me online, and we'll find a server and tear it up. I'd like to get a nice group together and we can start wrecking the Czech kids at CTF. Someone has to dethrone them from being the best Doom CTF players
Check out their website for some more info, you can find it here.