If you get a message in game that looks like this:

"hi ! give two minutes to see this website : http://steamactivation.o-n.fr/ i, if you will activate your account you will win a lot of games like [ CS:S , DOD:S , SDK , COD4 .. ]"

Don't believe it!! They try to trick you into giving them your account and password, saying you'll get free games. Well, you won't, so don't listen to them!! Never give anyone your password!! They disguised themselves to look like Steam, but they're not so don't buy it. They're just trying to get your account. :[