Images for several Ubuntu flavours are available.

1. Ubuntu Linux 6.06.1 codename Dapper Drake
   Size (compressed/uncompressed): 458 MBytes / 2.55 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-6.06.1-x86-2.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: from standard (GNOME) ISO image, Guest Additions not installed (HOWTO)
2. Ubuntu Linux 7.10 codename Gutsy Gibbon
   Size (compressed/uncompressed): 512.4 MBytes / 2.7 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-7.10-x86-3.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: from standard (GNOME) ISO image, Guest Additions not installed (HOWTO)
3. Ubuntu Linux 8.04 codename Hardy Heron
   Size (compressed/uncompressed): 503.8 MBytes / 2.67 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-8.04-x86.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: from standard (GNOME) ISO image, Guest Additions installed
4. Ubuntu Linux 8.10 codename Intrepid Ibex
   Size (compressed/uncompressed): 511.6 MBytes / 2.68 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-8.10-x86.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: from standard (GNOME) ISO image, Guest Additions not installed
5. Ubuntu Linux 8.10 Server codename Intrepid Ibex
   Size (compressed/uncompressed): 193.5 MBytes / 1.15 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-8.10-server-x86.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes:LAMP Server and Samba Server are installed; MySQL root password is toor
6. Ubuntu Linux 9.04 codename Jaunty Jackalope
   Size (compressed/uncompressed): 560.1 MBytes / 2.58 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-9.04-x86.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: Guest Additions not installed; only .vdi is provided
7. Ubuntu Linux 9.10 (Alpha 1) codename Karmik Koala
   Size (compressed/uncompressed): 519 MBytes / 2.52 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-9.10-alpha-1-x86.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: Guest Additions not installed
8. Ubuntu Linux 9.04 codename Jaunty Jackalope x86_64
   Size (compressed/uncompressed): 798 MBytes / 3.60 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu_9.04_amd64.7z
   Active user account(s) (username/password): ubuntu/reverse
   Notes: Guest Additions installed, image courtesy of Rainer Posel
9. Ubuntu Linux 9.10 (Beta) codename Karmik Koala
   Size (compressed/uncompressed): 496,4 MBytes / 2.4 GBytes
   Link: http://downloads.sourceforge.net/virtualboximage/ubuntu_Karmic_beta_i386.7z
   Active user account(s) (username/password): virtualbox/virtualbox
   Notes: Guest Additions not installed
10. Ubuntu Linux 9.10 codename Karmic Koala x86
    Size (compressed/uncompressed): 478.47 MBytes / 2.40 GBytes
    Link: http://downloads.sourceforge.net/virtualboximage/ubuntu-9.10.vdi.7z
    Active user account(s) (username/password): ubuntu/reverse
    Notes: Guest Additions installed, image courtesy of Rainer Posel