There are a tonne of shopping carts out on the market, some of them free, some of them not. Miva Merchant comes in at the top end of products for small business, Zencart, and OScommerce. In many ways, each of these is kind of top heavy on the learning, powerful, and not particularly flexible, esp. in the hands of SMEs who perhaps don’t have so much experience.
However, Avactis Shopping Cart makes it very easy to integrate your online store into your existing website, thereby making Avactis an easy and obvious choice if you don’t want to redo your entire website around your store (many people wouldn’t choose this route!). You can view a site made with Avactis here.
I tried OScommerce and found it very clunky. Without experience of using it, it was quite difficult (impossible for me) to integrate this with an existing website. I also tried Zencart which was easier, but still not straightforward.