One thing that I forgot to add to the list is this nifty little free content generator tool for quiz's:
Multiple Choice Generator - ContentGenerator.net - create your own Educational Flash games
It creates a Flash file to embed in a HTML page.
I used it to add quiz's to the History Nexus MySpace page but it would be easy to add it to a Pligg page.
Myspace.com Blogs - History quiz number one - History Nexus MySpace Blog