Magento: Open Source & Powerful E-Commerce Software
Magento is an e-commerce software which is still in the beta version. On the other hand there are already many users loving it because of the way it works and its features.
I had known Oscommerce as the most popular open source e-commerce software since today but it seems like Magento may be the new one.
This is really a smart software which is standards-compliant, has templating, multilanguage features and many more. One of the features amazed the most is "you can manage more than 1 e-shops" from the same admin.
There is already an active community using the product where you can find for support too.
And, here is the admin screenshot. See how nice it is:
Frontend Demo: http://demo.magentocommerce.com
Backend Demo: http://demo-admin.magentocommerce.com/
Backend Demo - User-Pass: User: admin, Pass: 123123