I'm a fan of Munin. It's super easy to write custom scripts for it in any language. For example, here's a script written in ruby to graph Passenger memory usage: http://gist.github.com/37525

Munin is only one component of a complete monitoring setup however (good for observing long term trends in resource consumption). I also use Monit to issue alerts if a service is down, and Pingdom to monitor external service availability. Has worked well so far.