My two favorites are:
1. OpenCart – http://www.opencart.com
Clean and simple. May not have all the plugins you need however.
2. Magento – http://www.magentocommerce.com/
This is still in a beta form, but it looks AWESOME! I can hardly believe its open source.
I’ve given both OSCommerce and Zen Cart the boot. OSC is still based on old school table layouts (I think) and ZC is a friggin behemoth. It’s slow, especially on shared hosting.