steam beta must be running

"steam beta must be running to make use of find servers"
"error steam beta must be running"
"steam beta must be running mac"
"steam beta must be running tf2"

It's a pain, it's ambigious, and not only is the error message confusing, but it actually is wrong. But once you know the solution you might understand part of how this error shows up.

But it's so simple.

Most of the time this is fixed by restarting steam. The next best thing is to check for an update in order to update steam.

In fact do that first since it will restart if there is an update.

SOLUTION:
[Check for an update + Restart]
On mac this means going to the Steam menu at the very top of your screen and choosing "Check for Updates"
In windows you will find the check for update option under the tools menu.

If there wasn't an update, restart. If there was an update it will ask to restart by itself.

If neither of those work try validating your game files by right clicking on the game in your game list, choosing "Properties" followed by selecting the "Local Files" tab and you should see a button that mentions validation.

If all three of these do not resolve your issue it's time look for other causes for your problem.

What's happening:
The game you are trying to find servers for isn't happy with the version of steam you are running. So it either isn't up to date or something is preventing the game from properly shaking hands with Steam, hence the restart.

Forum threads to watch
http://forums.steamgames.com/forums/showthread.php?t=1306946
http://forums.steampowered.com/forums/showthread.php?t=824497