Ecover Chlorine Free Natural Oxygen Bleach keeps your whites naturally white, even after several washes. It quickly and effectively removes all kinds of stains.
We add a teaspoon or two of this (we have a front loader) to the machine along with our regular washing powder (we use Euca laundry powder) when doing a load of whites - and our whites always look bright and fresh and very white.
[Another secret for keeping you whites looking white is to wash whites only with whites or very, very pale colours. This will help to keep them from going a dirty greyish-white.]