Apache is a great web server, but it is not without problems.
One of those problems is that Apache can sometimes "hang" in a state where no pages are being served, but the Apache "httpd" processes are still running.
This can be very badm because most server monitoring systems won't detect this error.
m3rlin has created a pretty useful script for monitoring Apache for a specific error, running out of MaxThreads. The script is available at Automatically monitor and restart apache.
The script is an excellent framework for monitoring other service outage issues which can plague Apache.