Just want to add my recommendation for VirtueMart. I've been using it for just over a year now, and it's been great. The level of detail in this cart system is great, and the add-ons (postage calculators) for example really make this a powerful tool. Mahuti was suggesting in a post that it's difficult to customise the CSS. The open source community doesn't have the best reputation for great looking websites to say the least. But as a designer I decided to get into the CSS of the cart (and Joomla) and make it look good. Check out http://jstore.com.au if you like and see how it looks. These guys have done a great job getting a functional cart together, with a bit of styling the whole thing can work really well and look great.