There is a big choice of shopping carts all over the internet and sometimes it's rather difficult to choose the one.
In this article we will guide you how to choose a shopping cart.
One of the main features of any shopping cart is user-friendly interface for the shoppers. They should not guess how they can add products to the cart and purchase them.
People do not like filling the forms. Your shopping cart should not require the shoppers to be registered.
The other important thing is the shoppers should like the design of your shopping cart. It will be a big plus if the shopping cart is based on templates.
If you follow the simple rules above you can increase your sales.

Free shopping carts. Some people like free shopping carts others don't. Some developers of free shopping carts pay less attention to the security.
As a result a free shopping cart can have security problems.

The most popular free shopping carts are:
osCommerce www.oscommerce.com
Zen Cart www.zen-cart.com
Those shopping carts are rather powerful but from the other hand sometimes it's difficult to set them up.

Commercial shopping carts. They are more professional as a rule. The developers are more interested in developing them.
One of the commercial shopping carts is 1st ProCommerce (www.1procommerce.com). You can sell digital downloads and merchandise using this shopping cart.

More expencive commercial shopping carts are Cart32 (www.cart32.com) and ProStores (www.prostores.com).

P.S. This is my own opinion. If you have anything to add you can reply to this post.

For any tangible product they're going to have to provide a shipping address so why not require them to register and save the address so they don't have to enter it again.
If there's more to the site than just the shopping cart then the cart should look the same as the rest of the site but it doesn't matter how this is accomplished.
That doesn't follow. Open source has a lot more people looking at it than closed source and can be more secure.
You forgot cubecart.com
I found osCommerce difficult to modify but the other two are quite easy to install and setup.
The developers are more interested in making money.

When you set up a ready-made shopping cart system like osCommerce, you get a ready site, working on its own, I mean, it's rather autonomous. But what if you do already have a website and then decide to add the shopping cart functionality to it? How do you integrate the two different systems?
Currently I know only one cart software vendor that foresees that problem and offers a nice solution: it's www.avactis.com. Their cart can work on its own, of course, but it's also possible to insert different parts of it into existing site pages, by inserting understandable tags like <?NavigationBar();?> or <?ProductName(38);?> That way it's possible to glue the cart and the site together, to some degree. So Avactis is another choice.

For the same purpose (integration of 'cart' into your site) i would recommend you LiteCommerce.com software. It has checkout mode when catalog is disabled and you use it as a 'cart' only.

Also desing editing is rather simple for LiteCommerce, so one can just changes its checkout page and that's all.