Download ready-to-use OVA files containing your favorite OS, such as Debian, Ubuntu, Mint, FreeBSD, OpenBSD, etc. Simply "Import" the OVA into VirtualBox and go. (Control+I when in VirtualBox)
Most distributions come in their 32 bit and 64 bit flavors, as well as "Minimal" and "GUI" versions. Where possible, guest additions have been installed to optimize the experience.
Overall this is to aid in quick deployment, testing of environments, application development, or simply getting...