Best $20 Multiplayer Game

Terraria is a fantastic multiplayer game.

I would go for bad company 2. I have it for some time,and it's still pretty fun to play!

Bad Company 2 is still quite populated, WaW is not as much but still has some good servers around. Both are great games and would recommend getting them.

The two that you said, they are both populated and homefront is another good addition

bad company 2 and homefront, but there arent many players around today
cod4 is the best i think, but costs a bit more

BC2 of course

Dungeon defenders

L4D and L4D2

I'd say killing floor if you don't mind coop games...

Torchlight 2

any good rts. Nexus is fun. Dawn of war etc. Total war series(you can get medivil gold for 10 or so on gmg, activate on steam) look at my games and wish list.

Serious Sam, Killing Floor, Torchlight II (great Diablo clone).

RU CD Key for BF3!
You just have to download the language pack!

I would say bad company 2. I have hundreds of hours on this game and to be honest, I think I liked it more than bf3.

Shooter: Quake,
Shooter&rpg: Tribe Ascend (free, but not bad multiplayer),
RTS: Warcraft (not WoW) or AoE (No-steam),
Minecraft: Minecraft (No-Steam),
Walk&Kill: Alien Swarm (Free&Valve),
Simulator?: Saints Row: The Third (cool story, awesome things, but too much dlc),
Moba: Dota/HoN/LoL (they are actualy almost same, so.. It's only on you, with one you preffer, also free No-steam),
Zombie: L4D2/Zombie panic source (Both are actualy great),
Racing: TrackMania nations forever (free, but great racing game with multiplayer)

Killing Floor FTW !

Garys Mod.

Borderlands
Terraria
Killing floor

Torchlight if you like A-RPG (but T2, and you have the first one too)

also you can buy Arma II, and play the DayZ mod i you like the zombie survival hardcore stuff

Torchlight 2!!!!

Call of Duty 4; Modern Warfare, easily the greatest CoD game and one (if not) my favorite multiplayer game....,ever. Everything just works SO well! Guns are balanced to perfection, maps are fun and memorable, countless mods exist, the players aren't 5 years old (or at least, they've all moved on to the later CoD games by now ;) ), and I've spent 700+ hours on it total (though 600 or so is from my PS3 time on it, from launch till 2009 or so), STILL playing it now haha. I loved the single player too, seriously.... I cried at one point even. :P

A absolute steal for $20, no joke. A great 2nd would be a copy of Garry's Mod for you and a friend (It's $10).....but you basically need HL2, CS:S, and the HL episodes to play it so idk. :\

Check out Firefall too. It's in beta though. BUT FREE.

Garrys mod is great, had a lot of fun playing ttt and other mods.

Also look into ARMA2, but get the DayZ mod, there is a steep learning curve, but it's crazy fun, it should still be on sale at amazon for $15, its not steam though.