Testing to see if site/app is up and running

[runner]

We monitor our web sites using ICMP and a "dumb" http port monitor. However, sometimes a back-end application, like tomcat or oracle, will die and the web server and port 80 are still reachable but the app is down.

How do you guys monitor your sites to verify they are up? Looks like a simple port monitor won't cut it any more. I guess a person could write a script to parse the apache error log; looking for certian errors but I don't know every error to look for.

Is there an open source monitoring tool that will not only check to see if the http server is up but will also log in using a test account?