http://www.avactis.com/
Developers of this new programme contacted me to design a
few templates..quite impressed with versions they supplied(1.71/1.72).
Still in early stages, but has some nice features not yet available
in zencart..not used OsCommerce in awhile so cannot compare.
Avactis is NOT free, but it is easy to integrate into an existing website.
Might want to give it a try via free download option..