A lot of our gateway customer use ekmpowershop which is a hosted solution. Other popular carts - with our merchants at least - seem to be OS Commerce, Actinic, Zen Cart, Roman Cart and VirtueMart. We are working on a plug-in for Magento at the moment as that seems to be a particular favorite of developers at the moment.

I think the thing with shopping carts is that each will have their plus points so it is up to you to decide which works best for you. There are quite a few open source carts available meaning you don't have to spend money to try them out.

Simon