Umbraco/Plugins And Extensions/Commerce4umbraco
Commerce4Umbraco
editShopping cart software for umbraco.
See here for more info: http://www.codeplex.com/commerce4umbraco or Paul Sterling's blog http://motusconnect.com/blog.aspx
The Umbraco Store is also based on the Commerce for Umbraco extension: http://store.umbraco.org