Skydome

Edit: As you will see in MacSlow's comment below, David Reveman has chosen to implement only .png images - so the quote from MacSlow's blog is now incorrect.

Well, I'm a bit late on reporting it, but here you go:

MacSlow has made a patch for Compiz (not strictly a plugin, but it fits here anyway ;) to allow the background of the cube rotation rendering to be changed.

A user can use gconf to set what is displayed behind the cube

MacSlow says about his patch:

My skydome-patch will allow you to set any power-of-2 image (e.g. 1024×1024) to be used as texture for the background (that empty void of space behind the cube, only visible when you rotate the cube). Filetypes like .png, .jpg, .tga and .bmp can be used. You can set the image on the fly via gconf-editor. If you enter an invalid filename a fallback color-gradient will be used. No more black backgrounds. Have a look at the screenshots below.

You can read more about it here
http://macslow.thepimp.net/?p=31