Thanks to extenders, it is now very easy for applet developers to embed relocatable widgets in your applet. Any QGraphicsWidget can be wrapped in a so-called "ExtenderItem", which allows the widget to be simply dragged anywhere by the user. Items that have been "detached" this way, can live on even when the applet that created them goes away, and can even be persistent between sessions. Widgets can also be reordered by the user, they can be collapsed and expanded, and custom actions can be registered for easy interaction. Take a look at the following simple screencast to get an idea about what all this means.
What is shown here is the kuiserver applet I made to test the extender API. It is located in KDE SVN playground at the moment (if you'd like to test it), but it will most likely move to kdebase for 4.2. The kuiserver can show any KJob, which can be file transfers (as shown here), but also the burning of a CD, the rendering of a 3d image - basically anything that takes some time. As you can see, the jobs that are shown can be moved around, detached (in which case they are persistent, so you'll see them again even after you restart your computer), and returned to its source (the little question mark icon, since there's no icon yet for "return to source"). Also, a couple of custom actions are registered by the applet to its ExtenderItems, allowing the user to pause, resume and stop its transfers.
One of the nice things about extenders is that they require very little work for a developer to use. Thanks to all the great feedback from the rest of the Plasma team, it has a very neat and easy to use API, where basically all you have to do is instantiate an Extender, and start adding ExtenderItems to it. For a more complete picture on how to approach extenders, you can take a look at the TechBase tutorial I wrote. It's still a bit of a work in progress, but I think it might be useful. The tutorial shows how to create a stripped down version of the kuiserver applet shown in the screencast.
So where to go from here? Well, there are lot's of places that extenders might be useful. To name a few: The notify applet. Imagine being notified of an important email, and being able to just drag that notification to somewhere on your desktop, or a ToDo applet on your panel, so you'll remember to answer that email once you've finished what you're working on. Or a system monitor applet, where you can simply detach the sensors that you are interested in. Or, since Plasma is all about clocks, a clock where you can detach different timezones. And once all kinds of applets start using this, you have the freedom to just drop almost any part of almost any applet together as you see fit. And that's the kind of flexibility Plasma is all about!