I've just written a tutorial on writing plugins that use the Engines 1.2 plugin. The latest release of Engines changes quite a few things, and while they're all documented in the rdoc, I've pulled them together into a more easily-digestible format for beginners.
It's at http://gwyn.dezyne.net/page/writing_a_plugin_with_engines_ .
I've also released a plugin called Macaque, which is a brutally simple wiki. It's at http://gwyn.dezyne.net/page/macaque .
I'd be interested in feedback on either or both.
Cheers,
Gwyn.