~S~ is used in chat for online multiplayer combat games for saluting or addressing many people in a room. It means "salute (all)". As opposed to S! for saluting.
It is probable that this originated in Microsoft Combat Flight Simulator and moved up through the years to other games and newer versions such as IL2 or CS.
Maj_Ojo: ~S~ all
AH_Hornet: S! Ojo
AH_BULLET: S! Ojo. Hey bro.