Every year I do a round up of top PHP carts but to my dismay they are all pretty bad in their own ways - Xcart, zen cart, and oscommerce seem to be the most popular, but they all pretty much stink falling short on either extendability, customization, and usability. Oscommerce is the only one that that offers a HUGE library of contributions that allows you to extend or write your own extensions which makes it our top off-the-shelf choice. PinnacleCart would be my top hosted choice.

If you want to spend some money (it takes money to make money), then move to ASP - you have way better choices for ecommerce - Storefront is good and ProductCart by Early Impact is also really good.

For us, we have discovered that there is NO one-size-fits-all ecommerce application which is why we write our own custom applications to fit the type of business.