Many may not know about this wonderful website. I dont remember how I came across this one either. The site is:
http://opensourcecms.com/

This site has demo installations of various open source packages/applications. Before we try installing any on our own site, we get to try out these packages on this website.

Here are some categories:

• Portals (Drupal, Joomla, Mambo etc)
• Blogs (Wordpress, Lifetype etc)
• E-Commerce (osCommerce, Zen cart etc)
• Groupware
• Forums
• e-Learning
• Image galleries
• Wiki

Ofcourse, you will have to deal with some ads. But they are not so distracting.