A neat feature that was quietly added to GNOME 2.22 is time-lapsed backgrounds; essentially a nice slideshow for your background image. Works best with very long transitions and very few keyframes.
This immediately had me thinking about the Dawn of Ubuntu wallpaper, and its two variants Night of Ubuntu and Ubuntu Spring. I popped them together into a little slideshow, and am quite pleased with the results
This slideshow ("Day of Ubuntu") gradually changes over time with a fading transition, continually providing a unique view that perfectly reflects the time of day!
Unfortunately, the images cannot use relative paths at the moment, so you have to place them as directed. Copy day_of_ubuntu to /usr/share/backgrounds (sudo cp day_of_ubuntu /user/share/backgrounds/day_of_ubuntu), then add day_of_ubuntu.xml as a wallpaper.
Read Readme for more information.
(Also unfortunately, it seems that I must set an arbitrary day, month and year, which means that the slideshow gets messed up over time).
Again, please note that this is for Ubuntu Hardy; it will not work in a previous release, though should work with any recent GNOME based distro. Check your GNOME version in System -> About GNOME.
I like it, so I hope you do too!
Added midnight scene, entering after 9PM, in full force at 2AM. Sorry, the times here probably make no sense on other lines of latitude...