<p>I would like to be able to monitor my major system health indicators from inside our Java-based system.  Major points of interest include CPU temperature, motherboard temperature, fan speed, etc.  </p>

<p>Is there a package available that:</p>

<ol>
<li>Makes this sort of data available to Java?</li>
<li>Works on Windows or Linux or both?</li>
<li>Is open / free / cheap?</li>
</ol>

A few months ago I looked for such a library and I found nothing interesting. It is not impossible to create one, so I would recommend doing so. You'll probably need to access native libraries, to do that use
JNA
(it's easier than JNI). Start by adding support for a few things on one platform, then start adding support for other features and platforms.
The share it with us! People will starts using it, maybe even help with development and soon you'll have a fully featured system monitoring library for Java.