Commerce for Umbraco is an ecommerce extension that runs entirely within an Umbraco instance. It is easliy extended to accomodate nearly any ecommerce scenario. Currently it is not compatible with the 'new' schema from 4.5 and later.
4 February 2011 - The awesome folks at ProWorks have taken over this project and are sure to update it soon.
To see Commerce for Umbraco in action visit some of these client sites: