Source’s multiplayer network code is time and gamer tested by millions of players around the world, with an average of more than 9 billion multiplayer minutes played per month.
Server browser and friends instant messenger.

Prediction Analysis and Server-Side Lag Compensation for reducing the visible effects of latency in the connection between clients and the game server.

Server Browser. Displays all active game servers and allows a player to choose which one to participate on. Players can filter and sort server lists in order to speed up the display and selection of a server.

Friends Instant Messenger. Allows players to message each other both in and out of the game, as well as join friends in existing games. No more confusion about what server friends are on or how to chat—the Friends Instant Messenger keeps everyone connected.