Free Multiplayer Games

Browser games:
Curve Fever 2 (surround/light cycles with powerups -- 6 max, 8 with a single premium account)
Haxball (air hockey meets soccer -- 22 max, teams only)
iSketch (free-for-all Pictionary -- no max)
EpicMafia (a speedier take on the classic forum game -- 25 max)
Town of Salem (epicmafia with less options, but visuals -- 7-15)
Bombermine (bomberman mmo -- no max, 4 per group)
World of Tanks (an mmo-esque tank battler -- max unknown, teams only)
Board Game Online (the most skill-based game on the internet -- 16 max)
Quake Live (free installment in the fps series -- 16 max)
Tetris Friends (free-for-all Tetris with optional powerups -- 6 max)
Pentolla (Blokus clone -- 4 max)
Puzzle Monsters (multiplayer puzzle bobble -- 6 max, teams only)
Transformice (co-op puzzle/platformer with randomly generated levels -- ~20 max)
Pretend You're Xyzzy (Cards Against Humanity, aka vulgar Apples to Apples -- 20 max)
PseudoBabble (form sentences from a word bank -- 12 max)
Tactics Arena Online (Final Fantasy Tactics-like strategy game -- 2 max)
Tempest Chess (ffa, not turn-based chess -- 4 max)
Quadradius (checkers on crack -- 2 max)

Downloadable games:
Supraball (fps meets soccer -- 12 max, teams only)
Starcraft Arcade (various minigames built in the Starcraft engine -- variable max)
OpenLieroX (real-time Worms -- 6 or 8 max)
Gunbound (a Worms-esque strategy game in which a single machine is controlled -- 8 max)
Hedgewars (a good substitute for those too lazy to set up the real Worms -- 8 max, teams only)
Hockey? (minimalistic first-person hockey with realistic handling and physics -- 10 max, teams only)
Golf? (a sandbox-esque golf game featuring golf carts -- 4 max)
Altitude (2D dogfighting
Team Fortress 2 (duh -- variable max, teams only)
Trackmania Nations Forever (a time attack-based racer with user-created levels -- no max)
Warsow (shooter using Quake engine -- max unknown)
Toribash (crazy strategy-turn-based-physics-sandbox fighter -- 2 max)
King Arthur's Gold beta (TF2 meets Terraria -- 16 max, teams only)
Super Smash Bros. Crusade (SSBFlash but actually solid; lag may be a big issue -- 4 max)
Pokemon Online (the #1 battle simulator -- 2 max)

MMO-esque downloadable games:
PokeMMO (what it sounds like - based on FRLG region and engine)
Pangya (a Mario Golf-like mmo [breaks the hell out of Skype] -- 4 max per party)
Spiral Knights (a co-op dungeon crawler -- 4 max per party)
Vindictus (hack-n-slash-style mmo -- 4 max per party)
Realm of the Mad God (an absurdly fast-paced top-down mmo -- no max?)