VirtualBox is now embedded in lots of package managers.
For instance, from Fedora 11, it is available in RPM Fusion repository.
Such a way, to install the open-source edition:
you install VirtualBox-OSE
For older version, it may be available in some no-official yum repositories.
Anyway, whatever your GNU/Linux distribution version, you should find a binary version from the official site (they cover Fedora, Red Hat Entreprise Linux, Mandrake, Debian, Ubuntu, OpenSuse ...).
After the installation, launch VirtualBox with your user (avoid launching it with root), and configure the storage of Hard Disk Drives and Virtual Machines (see preferences) to ensure having enough free space (by default, they are stored under your home directory).