Edit: It seems to be working seconds after this piece went live - just my luck - but then when I refreshed it was gone. It came back, so it might be OK for now, with intermittent problems.
I woke up this morning, and like the past few mornings, I hit up the Steam Store to see what games are on special during Valve's Steam Holiday Sales. Well today, I couldn't do that, because the Steam Store is completely down.
This could be because Valve is offering Left 4 Dead 2 for free right now - and its servers completely buckled under the immense pressure... but one would think Valve's servers would be equipped to handle that type of traffic. It should be back up soon, but for now, you'll have to keep tapping F5 waiting for it to return.