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Building
Besides regular Mac apps, Unity can build MacOSX widgets, using the Unity web plugin.
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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.
Publishing
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.
References
See the Apple Dashboard developer article for an overview of widget and links to more articles.