Trac encourages using plugins by providing an ever growing number of ExtensionPoints, where custom code could get hooked in to change/improve almost any aspect of the system.
You hardly could setup a custom feature-rich Trac application without using one or more of the Trac plugins from Trac-Hacks.org (t-h.o).
It helped me a lot, when I started configuring and customizing Trac.
It took a lot of my spare time to scan and test a great share of the plugins provided there to sort out the best suited for my applications.
Thanks to open-minded developers I finally became a developer myself. Even if it's sucking almost every minute of my spare time right now, I'm glad to know and have it arround, especially for the kind, skilled coders joining there.