I was experiencing a number of problems one morning. Mostly applications not responding. This included the Finder and Terminal. Oddly enough, Classic still worked fine. I became fearful that my whole system was on its way down, so I thought I would try something.
I learned that the shutdown ('sudo shutdown now') command in Mac OS X does not actually bring the system down as it does in certain Linux systems I have used. Rather, it brings the system down to "single user mode." As I understand this, most everything is stopped and the system is in essence stripped down to its bare bones. To verify this, I ran top while in this state, and counted only about four (4) processes running.
Now, I just typed "exit" and the machine exitted from single user mode and the Window Server started back up. If you try this for whatever reason, you may notice that the network and other items are started back up. It seemed to fix my troubles and my system was never taken totally down. This was evident from running uptime from the terminal to see the total running time of the system has been preserved. This is also a much faster fix than a full restart.
I am not a guru in the area of the guts of this system yet, so I can't be sure of what damage I am creating or not really fixing, but so far this seems to be a more favorable fix than a full restart.
[Editor's note: Anyone have any input on the safety of this versus a full reboot into single-user mode? I believe I read somewhere that if you use "shutdown" to get into single-user mode, you should still finish with a reboot and not just an exit back to Aqua, but I can't find the reference now.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020210114848431