If you're a fan of OS X's GUI but don't feel like paying for a Mac (or giving up your precious Linux), Macbuntu will install all the necessary apps, icons, and other resources to make your desktop look just like OS X.
If you're using Ubuntu 10.04 Lucid Lynx, just type the following four lines in a Terminal window, copy and paste it:
wget https://downloads.sourceforge.net/project/macbuntu/macbuntu-10.04/v2.2/Macbuntu-10.04.tar.gz -O /tmp/Macbuntu-10.04.tar.gz
tar xzvf /tmp/Macbuntu-10.04.tar.gz -C /tmp
If you're using the new Ubuntu 10.10 Maverick Meerkat, type the following instead:
wget https://downloads.sourceforge.net/project/macbuntu/macbuntu-10.10/v2.3/Macbuntu-10.10.tar.gz -O /tmp/Macbuntu-10.10.tar.gz
tar xzvf /tmp/Macbuntu-10.10.tar.gz -C /tmp
IMPORTANT!!!! MAke SurE yOu'vE in IntErnEt CoNNectIon!!!!
It's a pretty well put together theme, and it will even give you a few options during setup, like which logo you want to use in your menu bar, whether you want Firefox, Chrome and Thunderbird themes, and whether you want to add features like hot corners or an Expose-like window previewer. And, if you ever decide that you want to go back to the regular GNOME desktop, you can easily uninstall the theme by retyping the above commands, replacing./install.sh with ./uninstall.sh instead.
Macbuntu is a free download for machines running Ubuntu Linux. Thanks, Onetrack!