Space Sunrise is a pretty popular Plymouth theme which we've covered a while back. zeckman1138 has tweaked this Plymouth theme further and released "Earth Sunrise", a great Plymouth theme which you can see in action below:
The video is not all that good since it was recorded in VirtualBox, but it should be enough to get an idea on how it looks like.
If you're using proprietary nVidia/ATI graphics drivers you can use a script to fix the Plymouth!
Download Earth Sunrise Plymouth theme (includes .deb files for easy installation). It should work in Ubuntu 10.04+ and Linux Mint 9+.
Thanks to inameiname for the tip!