Suggest me some nice (cheap) multiplayer games from steam

I haven't actually played it myself but orcs must die seems good, also left 4 dead 2 is only 4 dollars.

try the ones that are for free :P

Payday 2?

DOTA is free.

Killing Floor. Not everyone has moved on to KF2 (I have.)

Magicka.

Left 4 Dead. Left 4 Dead 2.

Insurgency is supposed to be good although I've never played it.

Day of Defeat Source (I haven't played this so I am unsure of the community, but I really enjoyed the original day of defeat)
Insurgency (it is a mixture of cs 1.6/source & CoD)
Rising Storm Game of the Year Edition (high learning curve and slow paced)
Battlefield: Bad Company 2
Quake Live

You want COOP, or just online deathmatch or wha?:)

Team Fortress 2 ! =D

Tabletop Simulator

for $5 now you could get Left 4 Dead 2. You could go for Killing Floor 2 but the game is around $11 I guess

Down to One

cs:go

L4D2
Terraria
Risk of Rain
Hammerwatch
Orcs Must Die 2