On very rare occasions, the OS X Finder will lock up and not respond to keyboard or mouse actions. It happened to me tonight when I tried to move 300+ items from my iDisk to a SCSI SyQuest drive - I'm not sure if it was a bug in iDisk, the Finder, or the SCSI drivers. In any event, the Finder showed only the spinning rainbow, and clicking on it in the Dock revealed an "Application not responding" message. The machine was still quite usable (other open apps were doing fine and were fully responsive), but I wasn't sure how to resolve the Finder problem -- I couldn't launch any new applications (such as the Terminal or Process Viewer) since the Finder was unresponsive.
In addition, the usual escape route (command-option-escape) wasn't functional. The dialog box would not show up on the screen, regardless of which app was in the foreground.
I could have used another computer to connect and quit the Finder via "ps aux" and "kill", but that seemed like cheating. The only app I had in my dock (the only way I had to start programs) that wasn't running was CPU Monitor. On a lark, I launched it and noticed that it has two very useful menu items for troubleshooting system lockups - under the Processes menu, you can choose "Open Process Viewer" or "Open Top".
Either one of these was enough to solve my problem -- opening Top launches the Terminal, from where I could open a new window and use "ps aux" and "kill" to relaunch the Finder, or I could (as I did) use ProcessViewer to do it directly.
The moral of the story? To prevent a Finder lockup from rendering your machine unusable, keep one of CPU Monitor, Terminal, or ProcessViewer in your dock at all times. My personal choice is CPU Monitor, since I can get to either of the other apps through CPU Monitor. With one of these apps in your dock, you should (barring a dock lockup!) be able to launch a program to help you restart the Finder.
The nice aspect of working in OS X, of course, was that everything was back to completely normal once I restarted the Finder -- and, much to my amazement, I found that the copy that had locked the Finder had actually completed!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010814234911142