This morning I started experiencing problems with all of my OS X apps. It began with Safari not starting up, then I tried TextEdit, Console and others. Safari would not start up at all, TextEdit would start but then freeze after a moment or so later when I tried to access the menu bar; same with Console. Terminal would run fine, but when I started troubleshooting, I noticed that it would freeze when trying to change the Display preferences in the Window Settings.
I finally zeroed in on fonts being a possible issue and remembered that I had played with Font Book last night and disabled some fonts that I had duplicates of. One of these fonts was Helvetica Neue, which I realize now is one of the system fonts. Apparently many OS X apps rely on these fonts to operate. Enabling them with Font Book solved the problem.
[robg adds: This is related to a prior hint on problems caused by using Font Book's Resolve Duplicates feature. In both cases, a system font was disabled, leading to widespread system issues.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031104123352726