Previously on Ubuntu I has been using Qemu as a means to virtualize running PC sessions. This worked pretty well, and in Feisty the kernel module was available from the repositories (although, unfortunately not compiled). In Gutsy, you get something better. VirtualBox is now available straight from the Ubuntu servers, just start up Synaptic and search for the 'virtaulbox-ose' package, select it and you will get the kernel driver with it.
For people in the dark, VirtualBox is beautified Qemu. It's more user friendly thanks to the included GUI and some patches that are not in the Qemu versions.
After the download and install complete, you will need to add all people that need access to VM's through VirtualBox to the virtualbox user group (quickly done through the 'users & groups' tool from the 'Systems/Administration' menu). The kernel module is added automatically to the 'modules.conf' file so there is nothing left to worry about.
VirtualBox is great, it allowed me to run OpenSolaris (which was not possible in Qemu). Ofcourse any Windows release works just as well (and fast!), ideal for the rare desktop application that won't work in 'wine'. For me it's a great tool to test several Linux/Unix distributions without too much hassle.
One thing that VirtualBox seems lacking appears to be USB support, this could be one last reason to keep on using Qemu after all.

You know the one classic question in Linux on how to get a list of all the hardware of the machine, and the answer is to compile the output of several '/proc' entries and several 'lsusb|pci|*' commands. There is another nice tool, now available in the latest Ubuntu - lshw. Packages are available for both command and graphical environments (it will put itself in the 'Systems/Preferences' menu in Gnome).
I really recommend you give it a try to get a better and cleaner view on your systems hardware then supplied by 'HAL Device Manager', which is supplied by default.

By default Ubuntu is missing some packages that enable you to watch dvd or encode mp3 files and other every day necessities (at least, according to some people). Gutsy has a new package available called 'ubuntu-restricted-extras'. This meta-package will install all these things for you, it contains:

gstreamer ugly/bad plugins, MS fonts, Java 6, unrar, flash 9, LAME and dvdread.