Ever since I successfully installed Ubuntu on a laptop for the second time, I got a little more adventurous about customizing a few things in Ubuntu. I am quite impressed with what I did and I think they are essential to most Ubuntu users. My must-have list below is very basic. There are other must-have lists (read here and there) for advanced users, especially those who do a lot of multimedia.
One cool thing about Ubuntu is you can customize the startup screens. I am sure it can be done in Windows and Mac OS but it is not as popular as Linux where there are websites like GNOME Art that keep a collection of user generated artwork such as wallpaper, login screens, icons, and window border. Before you go look for the Hilary Duff GDM Theme, you need some tools that would load the artwork.