Updates to Team Fortress 2 have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

Source Engine Changes (CS:S, DoD:S, TF2, HL2:DM)
Fixed a case where servers would accumulate ghost players
Fixed bug with SourceTV and Replay causing player count to be off by one.

Team Fortress 2
Added lots of new Summer items, including the Refreshing Summer Cooler Key
Added a new achievement: Escape the Heat
Fixed a case where the client-side voting dialog would have an incorrect list of items
Fixed a bug where some items would not update their team colors correctly
Fixed a client backpack crash
Updated the Lo-Fi Longwave to be paintable
Updated the localization files
Get the full article at store.steampowered.com

So, this is why the servers sometimes goes down? xD.