It has started in this group as well, so I will warn you guys. Maybe you have received a chat invite from a player named 'Hi.' , and a message like this: Steam gives you 1 free game to add to your account! Just visit www.exampleofalinksee.com! <- This link doesn't exist, I just made it up, but the real link the guy may have sent to you leads to a phising site. DO NOT CLICK IT! As it immediately sends you a virus when you do, a spyware virus!
This is called Hijack, this 'hacker' uses this virus to get steam account info from your computer. Simply don't click it, because when your pc gets the virus your info will be sent immediately. If you already clicked it, alas... It will be handy to remove the virus afterwards. But when you log out, your steam account will be robbed from games, but also items for games like TF2.
If you're account is hijacked (AND you're sure of it), do not give up hope! There is a way to recover your account and get every stolen item back: Steam support.
For further info about this situation, or questions, ask the Admins.

~J007~