virtualbox.org

I was thinking, maybe we could post links to web sites with premade VB images, that is downloadable? Please fill in this list with web sites if you know any other site.

Here is a list with lots of open sourced OSes, such as Plan9, FreeDos, Haiku, Many Linuxes, FreeBSD, etc:
http://sourceforge.net/projects/virtualboximage/files/

Did you check the Linux Guests section?