Ubuntu Feisty Fawn is a great operating system, and I've already helped install it on a number of systems. Here are five customizations I make right away to make it a more pleasant system to suit my taste:

1. Install the Flash and Java plugins. Ubuntu already has packages for these, but I believe they turned on by default for licensing reasons. Go to Applications: Add & Remove Applications and search for "ubuntu-restricted-extras". This will install Flash and Java plugins, as well MP3 support and some nice free Microsoft fonts. Check the boxes next to the listing and click "Apply" to finish the job. The Java plugin installs some things into the menu system that I think I'll never use, so I remove the entries for them. To edit the menu, right-click on "Applications" and select "Edit Menu". The menu editor should be fairly intuitive, and allowing you to remove "Java Web Start" and anything else you want. (Note: removing the menu items doesn't un-install these applications).
2. Turn off Google Suggest. I find the "Google Suggest" feature in Firefox to be annoying. To turn it off, enter about:config in the Location bar and press enter. In the resulting interface that comes up, search for "suggest". The preference for disabling this feature will be displayed. Right click on "true" and select "Toggle". That's it!
3. Enter fewer passwords. The following tips make the computer less security, but more convenient. Perhaps this trade-off isn't for you. First, you can skip the initial password screen at a boot and go straight to a particular account. To enable Auto-login of a user, go to System: Administration: Login Window: Security. Check the box for Enable Automatic Login and select a user to login automatically. On laptops, I also like to turn off the default of having a password appear automatically when the computer is resumed. To do that, launch the preference editor with Alt-F2 and then gconf-editor in the Run box. Once this program launches navigation to apps / gnome-power-manager. Check the box next to lock_use_screensaver_settings. Now this password will be disabled whenever the password for the screensavers are disabled.
4. Play DVDs. Ubuntu can't play some encrypted DVDs by default due to legal restrictions in some countries. Go ahead and try to play a DVD. It will pleasantly notice that you don't have the all the codecs you need installed, and offer to install some helper software for you. However, for an encrypted DVD, this still won't be enough. After checking any possible legal restrictions that may apply to your area, you can run a simple command to install support for playing encrypted DVDs. Use Alt-F2 to open the Run box. Copy and paste this into the box: sudo /usr/share/doc/libdvdread3/install-css.sh. Be sure to check the "Run in Terminal" box and then press OK. You'll need to enter your administrator password, and then wait briefly for the installation process to complete.
5. Use F-Spot for importing digital photos. I think F-Spot is a nicer photo manager than the default gThumbs application. F-spot comes installed by default is easy to set to have it open to import your photos when you plug in a digital camera. Go to System: Preferences: Removable Drives and Media: Cameras. The importing command you need to enter is: f-spot-import %h.

Thanks to groggyboy on digg for the tip about ubuntu-restricted-drivers