Open Source Ruby On Rails Shopping Cart - a good example

It includes...

• A simple content management system with blogging capabilities
• Manage your entire site's content from the web - no html necessary!
• A simple shopping cart that's tied into Authorize.net and PayPal (must have an account)
• Product and order management
• Track your inventory online
• View up to the minute sales reports
• Integrate with fulfillment houses via XML and web services
• A stunning administration interface
• Create and maintain content
• Create, maintain, void orders
• Answer questions from your visitors

Additional reading

Ruby On Rails - all blog postings related to Ruby on Rails
Ruby on Rails at Technorati - see what bloggers are writing