What was the first online game you ever played?

Ragnarok online and Gunbound

Mine was Unreal Tournament on Dreamcast

Yahoo! Games.... pool maybe?
Neopets
Does World of Darkness chat RP count? Never got into TT or LARP.

Command and conquer and warcraft where you had to connect to one of your friends computers via dialup to play together :D
Truly online - first one for me was Astonia.

Heroes might and magic 1 and diablo 2 and a lot more like command and conquer etc... all that good old games, nothing compared with the actual shit with nice cinematics and garbage gameplay.

Action Quake 2

Might be some game on a flash game site or so. Some of the first real AAA games wouldve been Battlefield Heroes and Battlefield Play4Free

Quake 3 an Tony Hawk’s Pro Skater 3

Guild Wars 1. :D

Ultima Online

Runescape 2 :)

I really don't remember, of course it was the '90s.. I can just remember how many years I spent on Ultima Online back in those days! =P
56k modem, the best we had here in Italy xD I recall I had to switch it on before going out to eat dinner..

Over the internet? Probably Team Fortress 2.

Over a local LAN? Diablo II. We played the shit out of this at work in our lab.

Two Worlds... yeah, I was just looking for a way to get a bit more fun out of an ill advised purchase.

Edit: I played Doom Deathmatch over LAN too if that matters.

I dont remember my first mp game, maybe Starcraft

Runescape. Free. I had fun.

Unreal Tournament or Counter Strike 1.6 :)

battlefield 1942 and medal of honor...good ol' gamespy era...

quake 3 arena as well...

Quake 3 arena

Not including web games like Club Penguin, for me it would be I think either Mario Kart Wii or Super Smash Bros. Brawl.

But the first one that really comes to mind is Call of Duty: Black Ops on the Wii. Unlike the above games, it had a whole friends system where you could add friend, send messages and even voice chat. I really enjoyed it and part of the reason I would play it so much is just so I could connect with the friends I found through the game.

I don't know why exactly, but I don't really even feel like I'm able to get that sort of experience on online PC games. Maybe it's because the player base is larger, maybe there is a greater focus of playing online with friends made outside of the game.

If online includes a LAN, then GTA2.

Marathon. Life-changing experience.

Counter-Strike 1.6 or Lineage 2 I guess. Third game for me was World of Warcraft if I'm not mistake

Ragnarok Online

It was probably a trial of Star Wars Galaxies that was included with an issue of PC Gamer magazine. So either 2003 or 2004, but I know it was before the Combat Upgrade (which occurred in 2005).
Funnily enough, somehow, I was invited back to play for a limited amount of time after the Combat Upgrade. I want to say it was around December, so maybe it was part of some holiday promotion?