qemu

VirtualBox is a very practical application running under Linux systems, and packaged into Ubuntu 8.04 Hardy Heron, that lets you virtualise an operating system. A bit in the same way as QEmu did it, except that, to make it short, it works better. Anyway, VirtualBox is highly dependent on the kernel of the base system you're running it on, so it is linked to the kernel packages in Ubuntu. The only problem is that, when upgrading your kernel with the usual Ubuntu package updates, you don't get an update for the VirtualBox right away.