"Unlike plugins that run for a specific person, everyone can see and interact with the same widget. You can add as many widgets to the board as you need and even run them at the same time. This makes them great for collaboration!" (Figma widget docs)

You'll find them in a new little menu on the nav bar, or by using the Shift+I shortcut:

Here's where to find out more:

• Figma community widgets

• Widget Documentation (build one!)

• On the Figma announcement twitter thread