Umbraco/Plugins And Extensions/Commerce4umbraco

Commerce4Umbraco
Shopping 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