Take NavHistory. Dynamo actually has a component which keeps track of the state of the user's history, and can display that in the form of breadcrumbs. It's mostly used in the context of an e-commerce site, but it's in straight DAS, just hanging out.

Or the whole concept of pipeline managers. You can manage any pipeline in a single XML file, and instantiate classes or components down that pipeline, for any process you want. I just pulled apart the parsing code for this blog and rearranged it a bit, and now I have more control over the post's internal structure and can do things like syntax highlighting in code segments.

Or the Performance Monitor. Everyone always forgets the Performance Monitor because it's stuck in the administration guide, but it should be the first thing to add whenever there's a question about how expensive an operation is.

This isn't even mentioning the supreme usefulness of the DynamicBeans package, which lets you call getPropertyValue() or setPropertyValue() on anything (not just JavaBeans -- you can set up your property mapping).

Will add more stuff as I remember it.