Zen Cart started as a fork of the osCommerce project and we tried it early on and, well, didn't like it.  However, much has changed since then and we would now recommend Zen Cart as an excellent choice for many of our clients.  

"Zen Cart™ truly is the art of e-commerce; a free, user-friendly, open source shopping cart system. The software is being developed by group of like-minded shop owners, programmers, designers, and consultants that think e-commerce could be and should be done differently." - Zen-Cart.com

Some of the 'out-of-the-box' features of Zen Cart include:

• Multiple customer modes
• Unlimited category depth
• Multiple sales and discount methods
• A template driven design for easy updates and look and feel modification
• Unlimited 'extra pages' for details about your company or organization
• Multiple shipping and payment options
• Discount coupons, gift certificates and more