Well, obvious answers would be:
- Blizzard 'Legacy' games such as WarCraft 3, Diablo 2, StarCraft 1
- World of WarCraft
- Some Valve 'Legacy' games (and some mods) such as Half-Life Deathmatch, Team Fortress Classic, Counter-Strike, Day of Defeat
- Team Fortress 2
- Left 4 Dead 1 & 2
- Runescape
- Ultima Online
- EVE Online
- Allods Online (one of the first and very rare "WoW Clones" that still survives to this day)
- Some games from the Total War franchise (Rome, Medieval 2, Napoleonic Wars, etc)
- Star Wars: The Old Republic
- Mount & Blade Warband
- Battlefield 1942 / Battlefield 2
- Alien Vs Predator 2
- Fightcade.com's service, being able to play many old(er) / retro Arcade games online today
- Tribes 2
- Command & Conquer 2 (Tiberian Sun / Firestorm) / Renegade, can still be played online (requires third party stuff, but can be done)
And many others I'm not seeing from my quick Google search, or that I'm not thinking about. And none of this even counts the very niche, sometimes very small communities that find themselves online to play old-school and retro games online via emulators for the NES / SNES / Genesis and so on (at least for those emulators that allow online play, and there's a good number of them by now).