Many widgets are already widely used: YouTube and iGoogle are popular examples. This one widget from Random House is an interesting initiative. Here is a brief summary of their announcement:
The widget is a lightweight internet application that can be easily embedded into a Web page. In its small version, at 195 x 335 pixels, it fits snugly into any website and delivers interactive book previews by allowing a user to page forward and back through the book or search for keywords in the actual text content of the book - without ever leaving the Web page.Now, you can let your mind wander and examine what new applications can be delivered as widgets to take advantage of web 2.0 functionality.
Here is an example that shows its potential.