When there are "insufficient resources" to log on a user

I have recently been dealing with the problem that several shared computers are refusing to log users on, citing "insufficient resources." I found that rebooting fixed the problem every time. Evidently, this error appears when there are so many users logged on (i.e. people just locked or used "switch user" instead of actually logging off) that the system is truly low on resources. Of course, it's not easy to figure out who is logged on if you can't get in, but a reboot kicks everyone out and fixes the problem.