There are a lot of DoD:Source servers still around. Maybe you have your filters set wrong on the servers listing in Steam?

http://www.gametracker.com/search/dods/?

TC Ghost Recon AW 2

AvP 2010

Frozen Synapse, i miss playing MP on that game so much.

magicka R.I.P, chivalry R.I.P, Torchlight II R.I.P, in other words 90% of steam multiplayer games are dead

Skydrift. Although there is a small dedicated community. Sometimes you just need patience to set up matches.

Is GTA4 multiplayer dead?last time I checked it wasn't doing so well...

Battlefield Bad Company 2 is dead in USA 8-10 servers with few players. In Europe still quite popular, I log in there and play with lag.

Spec Ops: The Line.
The multiplayer is actually great, but nobody plays it.

battle for middle earth 1&2

tribes 2

Last time I played Nuclear Dawn it had quite a lot of people. o.o

Alien vs. Predator 1 & 2

Neverwinter Nights (the original one. yes the DOS one, not the 3d one)

NOX

Worms 2

Team Fortress (the first one, as in the Quake mod)

Star Trek: Elite Force

Dark Reign

Could probably list a few more if I wanted to, but I'm lazy.

Booster Trooper, Sniper elite

Pong :(

TF2

i didin't know borderlands 2 is dead

because i played borderlands 1 and still find a lot servers

Since when Borderlands 2 is MP? is coop........and also if MP games is great, he won't be dead.

A shame that Sanctum and Nuclear Dawn have so few players.

Battlefield 1942, Battlefield 2, and Tribes: Ascend. All empty here in Aus, with only 1 server populated in tribes.

World of Warcraft.

Don't bother trying to correct me. Just let me live in my little world, ok?

Brink and Natural Selection 2

Dungeon Keeper 1 and 2!!

Wolfenstein Enemy Territory is near-death for me, sadly. No one plays original maps anymore :(
Also Bloody Good Time is dead too. There are only 3 servers and I didn't see anyone online.

I would happily play Rails Across America or Monopoly '95 online if their entire "communities" weren't 99.9999% dead.