A neat monitoring agent that will run inside your Drupal PHP engine is New Relic.   With New Relic we can monitor the PHP application performance as well as the overall health of the Drupal site in terms of under-the-hood metrics: database performance, error rate, response time, sluggish modules/APIs (it can locate these down to the line of code causing the site to spin at a zillion CPU cycle a second.  So, it is a handy tool to have...