Magento

Magento Magento is a big, feature-rich package. It is open source and free to download and use. There are optional subscription plans starting at $500.00 per month for enterprise operations requiring top level support. The coding is PHP 5 and is clean and easy to work with. It supports all of the features you would expect, and need, from a shopping cart.

Magento has the most features such as Google Checkout, product import/export, and order editing. There are far more features available than could be listed here. See the website for more details. Magento also as the cleanest coding of all of the packages I've looked at making it the easiest to edit, modify, and extend, although the learning curve will be steep even for those with osCommerce, or derrivitive, experience. It also has an excellent template (or skin) system making it very easy to change the look and later upgrade.

Although this is the newest package (at time of writing) and, therefore, has not been tested as much as the others, is is designed so patches and updates are easy to install. Magneto has some pretty significant hosting requirements so you will not be able to use it on the typical small business web hosting package. You will need a semi-dedicated VPS server account. There is a Magento partner, Crucial Web Hosting that offers a suitable package for about $25.00 per month.