Depends on what you want to do. If a standalone shopping cart site is what you want the have a look at something like osCommerce, Open Source Online Shop E-Commerce Solutions. If you want to combine Jooomla! and a shopping cart then look at Joomla extensions like Joomla! Extensions Directory - VirtueMart there are other options, Drupal has ecommerce modules at Modules: e-Commerce | drupal.org

There is so much choice out there it is hard to be general and pick something, you'll find other contributers here have their preferences that suit their particular needs. It is best to research the options, then download and play with them and see which ones suit your needs.