At my latest gig, Ideum, we've been doing some serious tinkering with widgets. We're interested in them because they provide a direct way to view great collections of images over the web, using a small footprint out of the browser. Today we launched a widget that displays the latest image from the National Science Foundations Media feed. You can check it out at: NSF Latest Image. The Solar Viewer has over 39,000 downloads and the recently released Solar Viewer Mini has over 13,000 downloads.
I think these widgets are popular because the content is compelling, if not stunning. I am currently finishing up Donald Norman's book, Emotional Design: Why We Love or Hate Everyday Things, and the widgets represent the ultimate in visceral design. I wonder if the images displayed in the widgets evoke a response at a more personal level, or what Norman calls the reflective level. For me, I think its the visceral appeal that draws me in. The sun just looks too cool!