A couple of important warnings in this regard. First, Apple has paid very little attention to Dashboard widgets in the last little while. So some of the widgets available through this site may not work (or may not be available when you click the Download link because the host site has gone out of business). In some cases widgets don't work because they require Adobe's Flash media player, which isn't bundled with Mountain Lion. Others are just really old and don't work with the latest version of the Mac OS.
Because some widgets are so old, you may see a warning indicating that the widget is from an unknown developer and informing you that you can't download it. This is all part of Mountain Lion's Gatekeeper security scheme. When you install Mountain Lion, by default you can download and open applications obtained via the Mac App Store and from developers who have registered with Apple. If you attempt to open an application or widget obtained from a developer that's not registered with Apple, you receive this "Nuh-uh" warning.
If you're desperate to have one of these forbidden widgets, open the Downloads folder (found in your users folder and in the Dock), hold down the Control key, and click the widget you've downloaded. Choose the Open command from the resulting menu. A dialog box will appear that includes an Open button. Click that button, enter your account's user name and password, and click Install.
To completely remove third-party widgets from the Add Widgets screen, hold down the Option key and the icons will wiggle. Click the X that appears in the top-left corner of a third-party widget to delete it. (You can't delete Apple's built-in widgets.)
About Web clips
Dashboard offers an additional intriguing feature: The ability to create widgets based on website content. These special widgets are called Web clips and they work this way.
Launch Safari and navigate to a webpage that you visit frequently. For example, you might check a favorite news or social networking site each day. Choose File > Open in Dashboard. A purple bar will appear beneath Safari's toolbar, and the page will dim. Move your cursor over the column or element that you want to capture as a Web clip and click. A rectangle with positioning handles will appear. Drag those handles to resize your selection.
Now click the Add button in the purple bar above. The Dashboard screen will appear and display your selection as a Web clip. Any links within that clip are live, meaning that you can click them to launch Safari and move to the linked page. In addition, when the content on the host webpage changes, so will the content of your Web clip.
Sign up for CIO Asia eNewsletters.