All kinds of programs are being created lately, presenting you with dynamic content, such as the Dashboard application being worked on.
How about a "scrapbook desktop" that pulls pictures and backgrounds from various web sites you were visiting?
You could have an option to make it automatic (from your cache, based on image size/dimensions, etc so you don't get ads or widgets), or drag a picture onto your desktop and move it around until you lock it there like SuperKaramba applets. The picture would also have associated metadata (URL, date, time, last search query, etc).
The metadata could be accessed by clicking on the particular area of the desktop associated with the data while holding down a hotkey. You could launch web pages, IM conversations with the appropriate history pulled up, launch a related music clip, etc.
The use of this is you could easily remind yourself of different things, have a place to put data temporarily that you want to remember (kind of like an extended Klipper), you can also "clear" the scrapbook desktop and start over, have different desktop scrapbooks for different days (kept in a cache which you could clear and limit the size of). The content on your scrapbook desktop could fade dynamically as the date changes, and you could have a "fuzzy scroll" bar that could bring you back in time or forward in time.
Another improvement could be informational taskbar buttons. Taskbar buttons could contain dynamic data about the application such as download progress, compilation progress, some sort of statistics or notification.
This could of course be turned on/off in the control menu for each window because perhaps all of the possible blinking and abuse of such a feature. On grouped taskbar buttons, the last button updated would have its content visible.
Sort of like a "taskbar notification area" but to the next level, with actual program data being shown. The background of the button could show a minimized screenshot or a statistics table, or the progress of a graphic being rendered in thumbnail form, progress of movie playback, etc. Many possibilities.