The entire menu bar (the white one, that holds the Apple menu, etc) has vanished on my iMac. In this instance, I think it went away when VLC crashed, but it's happened in other situations as well.
Any open windows still respond, but I can't right-click on the desktop or launch anything from the dock. The dock is still there, and I can click on icons to restore minimized windows, but clicking on something that isn't running, won't run that program. System-level shortcuts like Command ⌘-Tab and Control ⌃-F2 don't do anything, but I can still hit the Exposé key F3.
If I reboot, everything is fine, but of course it can happen again when something goes wrong. If this were a Linux box, I'd restart the X server. If it were a Windows machine, it would be like explorer.exe crashed, and I'd have to kill and restart that. Is there any analogous fix that doesn't involve waiting for the whole machine to reboot?