Here is an email I recieved Sunday at 6:30pm.

It seems steam has a bunch of Authenication servers offline or running maitnence. Some people can connect to any servers fine but most people are having problems at the moment trying to connect to any Half-Life or Half-Life 2 game servers. Ive sent a request to Steam notifying them of what we have seen and believe they are now aware of the problem. Once the Authenication servers come online just try rebooting your game server and everything will be back to normal.

Just so you understand the auth servers are what all clients connect to before connecting to any online server. Even if the server is online and the auth servers are down then you will get an error when trying to connect. I know some auth servers are up but believe they are maxed out thats why you get the error somtimes or most of the time because they are at capacity.