Julia's netbook runs Ubuntu Netbook Remix 9.04 (UNR's 9.10 installation process has changed, and doesn't work as well). Her teacher recently recommended http://arcytech.org/java/, which offers a bunch of educational games. Unfortunately, UNR doesn't include Sun Java, and getting it running was non-trivial.

The short version: I needed to enable the multiverse & universe repositories in synaptic, and then "apt-get install sun-java6-plugin". I initially installed just the JRE, but Firefox needs the plugin too. I also put the JVM's path in /etc/jvm, but I'm not sure if that mattered.

Interestingly, I had a similar problem at work last week -- CentOS 5 systems naturally don't come with Sun Java, but installing the JVM was easy. For both CentOS & Ubuntu, most of the documentation on installing Java (including Sun's) stops at getting the JVM installed, and neglects the Firefox plugin. On CentOS, I just dropped a symlink to /usr/java/default/... into the right directory under /usr/lib/mozilla.

Be sure you install for the correct version of firefox (some of our systems have bits of several different versions); if not sure, link the plugin into ~/.mozilla.