You already know PackoXsplash HD (we wrote about it a few days ago) which is my favourite Xsplash, but I guess not everybody likes the same things, so here are some other nice Xplash Themes to use with your new Ubuntu Karmic Koala 9.10.
Before we get started, here is a quick info you'll need. To change you Xsplash theme, download the archive for the Xplash theme you want and extract the contents to the /usr/share/images/xsplash folder (you need to open nautilus as root: Alt + F2 and type: gksu nautilus).
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http://www.webupd8.org/2009/11/6-nice-xsplash-themes-for-ubuntu-karmic.html