This plugin does a HTTP GET of the tomcat status page:

/manager/status?XML=true

It checks the resulting XML for:

* Low free memory
* Excessive thread usage

Note that although this sounds quite useful, the nature of garbage collection
in Java means that this plugin is prone to 'false positives' when reporting
memory usage. IE: it will complain about low memory, which may be fixed by the
next garbage collection.

I suggest setting the critical threshold for memory quite low - 50 Mbytes or
less.

It is also useful to set the 'max_check_attempts' higher than usual to give
garbage collection time to kick in.

This plugin has proved useful in debugging, by picking up when a problem
started, as distinct from when the tomcat actually failed.