Besides regular Mac apps, Unity can build MacOSX widgets, using the Unity web plugin.
The widgets are just a web player wrapped with the widget HTML. If you right/option-click on the .wdgt file and select Show Package, you'll see the generated Unity file and accompanying HTML, CSS and images. Any additional files loaded by the widget can be added in the widget. For example, this widget streams a movie, cat.ogv.
You will probably want to customize the default Unity build by replacing the Icon.png file, which is the icon that displays in the Dashboard dock. Version information can be adjusted in the Info.plist file. You can customize the back panel by modifying the HTML and CSS files.
Most widgets are submitted and listed on the Apple widget download page. Most widgets are free, though some are listed as shareware. If a widget is listed as a Staff Favorite or Featured Widget, it certainly ends up in the Top 50.
Widgets published include HyperBowl Classic, FuguBowl, FuguMaze, FuguTilt, FuguFlip, Fugutype...
Other Unity widgets include 3D Paradise Paintball, MacPinball, Banana Warehouse and Blingy.
See the Apple Dashboard developer article for an overview of widget and links to more articles.