(By the way, did you know that there are similar apps for DataEngines and Wallpapers? plasmaengineexplorer and plasmawallpaperviewer.)
Marco worked up a small proof-of-concept app in playground some time ago that lets us run Plasmoids in windows slightly more satisfactorily. Here's a shot of a clock and the microblogging Plasmoids running with a Konversation window in behind and the pretty KRuner window nearby:
Right clicking on a Plasmoid gets you the usual suspects in the context menu along with a "Quit" entry that quits that Plasmoid. What's really cool is that they all run in the same process, keeping overhead down. Now, the goal for this isn't just to allow us to run Plasmoids in random windows on the desktop, but to provide an infrastructure for running Plasmoids full screen with Plasma Mobile in a way that is resource friendly but which also keeps the apps away from the main UI.
You can move the Plasmoids around, run them with or without a window border and background and more. It is currently in kdereview, and I'm busy polishing this up for 4.5. The name of this little wonder? plasma-windowed.
It currently knows about the following command line flags: -b, --border or --noborder to control the window borders (default is to have borders), -f and --fulscreen which starts the app fullscreen. This is just the start (it's only 368 LOC right now), however, and I have lots of ideas on how to make this even spiffier. :)
Now we can show Plasmoids on the desktop, in panels, on a dashboard, on a mobile device, on a netbook, on a screensaver, floating about and in Amarok. Soonish we'll have them on KDM, Kontact, Skrooge and KDevelop, too. Woo!