The PHP::Impact blog has a new post that overviews the plugin system that the Symfony framework has to offer.
If you want to use the power of the Rails framework without having to learn Ruby, then Symfony is the right framework for you. After spending more than 10 months playing around with Rails, I can say that Symfony is a great alternative to Rails for programmers who already know PHP.
He talks about what a plugin is, how its used by the framework, the simple installation process and a "top ten" list of some of the most popular plugins (including sfGuardPlugin, sfSimpleForm and sfControlPanel).