Magento is an open-source shopping cart solution built on top of the Zend Framework (PHP). It looks very promising–I can’t wait to get my hands on it. The beta is set to be released some time this month. Anyway, on their development blog, they’re looking for examples of usable, and interesting ecommerce sites, so head on over there and tell them what your favorite ecommerce site is!
http://www.magentocommerce.com/blog/what-are-your-favorite-ecommerce-sites/
UPDATE: Magento recently posted the results of their request for good ecommerce sites. It is a pretty comprehensive list. Check it out!.