It seems like that too many people wanted to claim their free Left 4 Dead 2 gift today. So many, that the Steam servers actually went down for a bit. The Steam twitter service didn’t respond to the issue; however, the problem doesn’t seem to be as serious, because most people don’t have trouble with the games they intended to play, but rather with the community features and the Steam Store.
You can see people complaining on reddit and every now and then when you try to connect to Steam you may get the “Error Code: -118” message.
Currently the Steam Store can still be accessed by many, but crashes every now and then. With the Steam holiday sale and the Left 4 Dead 2 free gift, it seems that Valve got more intention than they were prepared for.