There are few ways to create an online store:
1. Develop an e-commerce application from scratch.
2. Use a hosted e-commerce software.
3. Use a ready to use downloadable / installable e-commerce application.
1st option takes so much time & resources which is not suitable for many people.
2nd one has both positive & negative points. The biggest negative points are: you'll be limited with the presented features & it'll probably be paid.
With the 3rd option, there are great open source e-commerce applications that will help you to create a good looking & functioning e-store. And, get support from the community. Here they are:
1. Prestashop
An amazing open source e-commerce application with a lot of features & a sleek interface.
It enables you to create "special deals" (like price reductions, gift vouchers), tag products, sell virtual goods & service besides standard products & many more.
It is pre-configures for PayPal & other payment processors can be easily integrated. You can find integration & theme options in the product's active
community.
It requires PHP & MySQL to run.
2. Magento
This powerful e-commerce software has very strong features both at the user & the admin level.
It is multilingual & also supports multiple stores to be managed from 1 admin interface which is unique.
It has lots of marketing/promotion methods to make sales easier like "buy 1 get 1 free" or "multi-tier pricing for quantity discounts" & almost any other
possibilities you can imagine.
Magento is integrated with PayPal, Google Checkout and many other e-payment systems.
The system works with PHP and uses MySQL as the database.
3. osCommerce
osCommerce is probably the most well known & one of the oldest free e-commerce application.
It is multilingual, SEO friendly, has a lot of marketing options & supports almost every payment systems (officially or unofficially).
It works with PHP/MySQL. There are lotf of free & paid themes created for it to totally have a different e-shop.
4. dashCommerce
An open source ASP.NET e-commerce application that uses MSSQL 2005 as the database.
dashCommerce is multilingual & totally customizable with the theming support.
PayPal & a simple weight-shipping calculation provider is integrated by default.
Unlimited categories can be created & unlimited images can be added to a product (thumbnails are auto-created).
If you're looking for an ASP.NET shopping cart then you don't have too many options & this is a great piece of software.
5. Ubercart
Ubercart is an e-commerce platform for Drupal.
It is a feature-rich application that can sell physical goods, file downloads and even site access/memberships with auto-renewals.
It is installed as a module to Drupal & this makes it stronger as this makes it combined with one of the best CMS applications.
Ubercart works with many payment systems like Cyber Source, Authorize.net, PayPal & more.
Products can be imported/exported via XML & single page checkout feature makes shopping easy.
6. Wordpress E-Commerce Plugin
For Wordpress lovers, this plugin makes e-commerce very easy.
You can use all the great features of Wordpress, in means of managing the content & the plugin integrates to the system tightly which enables you to
take the advantage of it to maximum like: when you upload a new product, Wordpress update system pings all the search engine, etc.
Share-This plugin can be used for social networking support. If you are familiar with Wordpress themeing than it is very easy for you to create new looks for a store. This e-commerce plugin integrates with Google Checkout, Paypal, Authorize.net, Payment Express and many other trusted payment processors.
7. VirtueMart
An e-commerce system for Joomla & Mambo.
It has many functional features from stock management to instant currency conversion.
Any number of images & documents (like user manuals) can be added to a product. VirtueMart supports live credit card processing and popular payment gatways.
With the help of the extensible structure of these CMS applications, VirtueMart can have new features by creating new modules.