Page-Specific Widget Areas with Simple Page Sidebars
It’s not always true, but it is sometimes true, that plugins that start with “Simple” do in fact perform an important task simply.
Simple Page Sidebars definitely passes the test. It creates per-page sidebars with a very simple and intuitive addition to the WordPress page editor, and to the “Reading” screen (which is where you set the default/fallback sidebar for pages where you don’t specify a custom sidebar).
I found Simple Page Sidebars very useful on an ongoing client project that needs a “Testimonials” sidebar only on a single “Work We’ve Done” page. As a note, though, the plugin doesn’t by default work with either posts or custom post types.
Before you start registering widget areas using complex custom rules, have a look!