GameRanger is a free download for Mac and PC that lets you play over 600 games and demos online with friends and opponents worldwide. GameRanger offers features like profiles, friends lists, instant messaging, in-game voice communication, and competitive ladders/rankings/ratings.
Created by Scott Kevill and opened to Mac gaming in 1999, GameRanger is the longest running multiplayer online gaming service on any platform. In 2008, GameRanger expanded into PC gaming receiving widespread enthusiasm and acclaim.
The list of supported games could be found here.
This is the easiest way to play with your friends online, A must have.