Moonroot brings up a window showing the moon at its current phase. It uses the X11 Shape extension, so you can put it on your desktop background without window borders and it looks like the moon hanging there on top of whatever background image you use.
It doesn't have many features yet; eventually it will let you change the date, and perhaps some other features. (Also it could use a better name.)
Download: moonroot-0.3.tar.gz.
If you want it by itself on your desktop, you'll have to tell your window manager not to "decorate" the window (no titlebar or anything). I also like to set it to be visible on all desktops. In Openbox, I put this in .config/openbox/rc.xml:
<application name="moonroot">
<decor>no</decor>
<desktop>all</desktop>
</application>