The Yahoo! Desktop Widgets Gallery Website was discontinued on April 11, 2012.
The Mac OS X and Windows versions of the Widget Engine itself are available on Ricky Romero's Website.
N.B. The Widget Engine runs on OS X versions up to and including 10.10.5 (Yosemite).
It does not run fully correctly on OS X 10.11 (El Capitan) or on macOS 10.12 (Sierra).
On Windows, the Widget Engine is known to run correctly on Windows XP and versions up to Windows 10.
On macOS Sierra (10.12.6), almost all of the functions of the Widget Engine can be used successfully, if the Widget Dock is kept closed.
If the dock cannot be closed from the menubar, the Yahoo! Widgets preference file can be edited (while the Engine is not running) to set DockOpen to false, so that the dock does not open when the Widget Engine is started.
The preference file is at ~/Library/Preferences/com.yahoo.widgetengine.plist and can be edited using BBEdit, TextWrangler and Xcode.
The canvas context.drawImage(...) function no longer works. It is also not possible to run Widgets with an open debug window.
Nevertheless, most/many Widgets can be used successfully.
Saturday, June 16 2018.