JMX system monitoring?

At work, we have a system called Sitescope that shows lists of various monitors that are red, yellow or green (in error, warning, or good). You can drill down into different groups and see which things are good and which things are slow or broken.

It also has an alerting system that sends email to a particular email addresses. We use that to email to our text messaging email address so we are alerted when our systems go down or become slow.

However, that system costs a fair amount of money. For my own stuff, I want to use a cheaper alternative that provides much of the same capabilities.

I know that JMX allows me to query and change things. However, I was wondering if there was a free or relatively cheap (perhaps $300 or less) system that would allow me to set up monitors and have them email me when certain monitors fail.

The app being monitored would be a java webapp running under Jetty (although I'd be willing to switch to jboss if necessary).