OpenCart is an open source shopping cart that is free! It’s really great and simple to use.

http://www.opencart.com/

Here is some detail from the website:
OpenCart is a turn-key ready "out of the box" shopping cart solution. You simply install, select your template, add products and your ready to start accepting orders.

It has order management and multiple payment gateways already built in and we provide lifetime free support and free software updates.

* Open Source
* Documentation
* Unlimited Categories
* Unlimited Products
* Unlimited Manufacturers
* Templatable
* Multi-Language
* Multi-Currency
* Product Reviews
* Product Ratings
* Downloadable Products
* Automatic Image Resizing
* Multiple Tax Rates
* Related Products
* Unlimited Information Pages
* Shipping Weight Calculation
* Discount Coupon System
* Search Engine Optimization (SEO)
* Module System
* Backup & Restore Tools
* Printable Invoices
* Sales Reports