If you are experiencing the crash at launch in TF2 or CSS on OS X 10.5.8 systems, add this to your launch options panel by right-clicking or control-clicking the game title in the game library listing and visiting the “Properties” panel, then clicking the launch-options button. In that text field add this item:
and dismiss the dialog.
This disables an engine change that was made to speed up game loading and reduce memory usage, but it doesn’t work correctly on all 10.5.8 systems. This is a temporary workaround until we adjust the engine code to avoid this conflict and ship the update.