1) Zen cart
Zen cart is one of the oldest open-source e commerce solution on the net it will give the user the complete ecommerce solution and with thousands of themes you can make a fresh look with, and do as little coding as possible. Add to that the power of all the plugins available tor this platform and you will have a very good choice for any web developer.
Zen Cart is so successful that most other ecommerce solutions in the market are using the same things, methods and even names as Zen-Cart does.
Prestashop is a fairly new solution and also a very powerful one it comes with a lots of themes that can be customize easily using smiley engine. These themes are ranged from different categories of the shop that they are designed to be used for. some with their own functionality and some with no other functionality than the one on Prestashop itself.
It comes with a lots of native functionalities and features that will cover almost everything that you need an e-commerce solution to have and well for everything else there are hundreds if not thousands of different plugins that will give you almost any functionality that you could possibly want for your site.
This Solution will give you ability to have your customers buy your products from where ever they are and any time and can be integrated into your current e-commerce solution. It will create an iPhone app that will display your products and give them the ability to purchase through it.
4) Tomato cart
I have never used Tomato Cart but from the demo that they have on their site Tomato cart is made to be different from all other solutions and they took it as far as when you login to the back-end of it, its look like the windows Desktop page as for its functionality well I have to say that it has very good functionalities as far as I could see and a lots of Themes as it is one of the good things about this software.
Yes I said WordPress and many of you are thinking this guy is insane WordPress is a CMS its not an e-commerce and you are right but the beauty of WordPress is its adaptability. Just install a WordPress plugin like estore and you have a simple e-commerce solution that is very customizable and will work perfectly.
And that is why I choose WordPress to be one of the top 5 ecommerce solutions on the net.