Zencart.
Zencart is a full e-commerce package. It is open source and free to anyone that wants to use it. Now if your thinking of setting up a professional online shop and the words "there is no such thing as a free lunch" and "you don't get something for nothing" are going through your mind then put them aside for now. Although that's generally true in my experience, Zencart is one of the exceptions. You can install it for free and there are no disabled areas that you have to buy. It has a massive following and support forum. There are hundreds of free expansion modules (some ask for a voluntary donation). Its probably the best e-commerce software I have used to date. I was so impressed that I set up this site to sell my designs and installation services.

Why do they do it for free?
Its run by a non profit organisation who believe e-commerce should be available to everyone, they rely on donations to keep the project running. If you wish to donate a few pounds to the cause that's entirely up to you, I personally make a donation for each shop package I sell.

Zencart Modules
Many developers have created all sorts of modules to add onto Zencart, you can find them here. Many zencart sites charge to install them, I have seen up to £75 per module. We at Shopzen Install them free of charge for our customers. These developers have spent their spare time developing very professional add-ons free to any Zencart user who requires them, they support them as well. We feel it would be an insult to them to charge you for their hard work, most of them can be installed in a few minutes. All we ask is if you use one of these modules you make a voluntary donation to the programmer directly, if they accept them. Or to Zencart. Here is the Zencart donations page

Can I modify Zencart?
Yes. Zencart has its core program and uses an override system that allows you to customise it to you needs. There are levels of customisation that will depend on your abilities or how much you want to spend. If you fancy setting up your own shop take a look at our standard package. We do all the installation work and set up the program for your business and host it for a year included in the price. Leaving you to design your own logo and configure your own colour scheme.

Is it easy to do?
It all depends on your abilities. You will need a good HTML editor, you can do it with a text editor. I use Dreamweaver myself, it handles the FTP side of things, has an easy to use editor and is excellent for modifying style sheets (css). If you use a third party template and you like it how it is, then its just a matter of replacing the company logo's with your own and mastering the admin interface. Because there is a massive following its rare to have a problem or question that has never been asked before, you can get instant help using Google. In the search box type zencart and your question. For example "zencart How do I change the Header Image" The answer is the first hit. You might like to take a look at the zencart site/forum http://www.zen-cart.com/