Updates to the Source Multiplayer games have been released. The updates will be applied automatically when your Steam client is restarted. The specific changes include:

Improved multiplayer voice communication, now using a much clearer codec

First release of a completely reworked in game text chat to which now includes a chat history and a filter to only see important chat messages
As a result of this update, all linux servers are showing "Server is out of date" until the host updates its Steam folders.