VirtualBox
Open Source Edition on Ubuntu 8.04 (Hardy)
Installing VirtualBox OSE on Hardy is even easier. From a terminal session run the following commands to install Virtualbox OSE.
sudo apt-get install virtualbox-ose virtualbox-ose-source virtualbox-ose-modules-generic
Add yourself to the vboxusers group. You can add more usernames after "whoami" if you wish.
sudo gpasswd -a `whoami` vboxusers
or
sudo usermod -Gvboxusers -a `whoami`
You will now have to log out of your desktop session and log back in order to update your group membership. Congratulations, you can now skip down to "Using Virtual Box" (which might be out of date?)
If you have a multi-core CPU and experiment high CPU usage even when the guest OS is almost no using CPU, you can force Virtual Box to execute in just one of your cores by launching it through taskset.
sudo apt-get install util-linux
taskset -c 1 virtualbox
See http://www.virtualbox.org/ticket/1233 and http://ubuntuforums.org/showthread.php?t=775503 for further info on this issue.