XDA member Dysgenic was apparently unsatisfied with his current Android boot animation - so like any good Android lover, he decided to do something about it. The result: an ever-expanding list of absolutely awesome boot animations. Just take a gander at these two (there are currently 8):
Installation is essentially comprised of three basic steps: first, download the animation(s). Second, rename the one you're using "bootanimation.zip." Finally, use ADB to push it and overwrite the existing animation. Alternatively, you can use an app to handle the dirty work.
I've yet to try one myself (although I will later), so I can't speak from experience here, but I'd guess these may not be silky-smooth on lower end hardware (hell, it's giving my netbook a hard time - yay Atom). Drop us a comment if you have any issues, and make sure to let us know what device you're using.
(Note: any questions or issues should be directed towards the XDA thread - they'll be able to provide much better support).