I must disagree. the background eraser is still very good even though its now quiet old and other applications have cough up.
http://www.campratty.com/4tooltours/kz/bge1.html
That porters post of the original pdf by Kris Z who created it.
Its quiet long but well worth a read. To Think Knockout was once more to buy then the entire PSP application.
All plugins that used the original Adobe SDK work in psp The newer sdk requires a licence from adobe, Who dont want other applications to catch up, no fear of that now corel has taken psp over