This weekend you can play Counter-Strike: Global Offensive completely free of charge on Steam, courtesy of Valve.
The extended demo is a great way to get into the game and if you like it, then you may even be able to get it cheap. Valve have said that if enough people try out the game on this free weekend, they may put it on sale next week.
Get on Steam now and try it out! The free weekend lasts right up until Sunday 1pm PST.