Here is list of Open Source Shopping Carts. All are PHP and MySQL based solutions. Forum members and posts I give, to see how active is community. When some cart is missing - let me know. I can add that to list.

• osCommerce
  • Website - http://www.oscommerce.com/
  • Latest release - osCommerce Online Merchant v2.2 RC2a  - 30 January 2008
  • Forum - http://forums.oscommerce.com/
  • Forum members and posts - 185746 Members and 1191276 Posts.
  • Demo - Demo at OpenSourceCMS

• Zen Cart
  • Website - http://www.zen-cart.com/
  • Latest release - Zen Cart v1.3.8 - 30 November 2007
  • Forum - http://www.zen-cart.com/forum
  • Forum members and posts - 62637 Members and 583097 Posts.
  • Demo - Demo at OpenSourceCMS
• cpCommerce
  • Website - http://cpcommerce.cpradio.org/
  • Latest release - cpCommerce v1.2.0 - 21 June 2008
  • Forum - http://cpcommerce.cpradio.org/forums/
  • Forum members and posts - 2311 Members and 16313 Posts.
  • Demo - Demo at OpenSourceCMS
• OpenCart
  • Website - http://www.opencart.com/
  • Latest release - 0.7.8 released 7th July, 2008
  • Forum - http://forum.opencart.com/
  • Forum members and posts - 1842 Members and 7489 Posts
  • Demo - http://www.opencart.com/demonstration
• PrestaShop
  • Website - http://www.prestashop.com/
    
  • Latest release - 1.0.0.8, 31. July 2008
  • Forum - http://www.prestashop.com/forums/
    
  • Forum members and posts - 2462 Members and 20842 Posts
    
  • Demo - http://www.prestashop.com/en/showcase_demo/
• Magento
  • Website - http://www.magentocommerce.com/
  • Latest release - Magento 1.1.3, 13 August 2008
  • Forum - http://www.magentocommerce.com/boards
  • Forum members and posts - 29680 Members and 67301 Posts
  • Demo - http://www.magentocommerce.com/demo
• IdeaCart
  • Website - http://www.ideacart.com/
  • Latest release - IdeaCart 0.02, 18 March 2008
  • Forum - http://www.ideacart.com/forum
  • Demo - http://demo.ideacart.com/ - not working right now
• Freeway
  • Website - http://www.openfreeway.org/
  • Latest release - Freeway 1.4.2.197 [Sathish]
  • Forum - http://forums.openfreeway.org/
  • Forum members and posts - 6044 Posts and 1180 Members
  • Demo - http://www.openfreeway.org/demo.html
• QUICK.CART
  • Website - http://opensolution.org
  • Latest release - Quick.Cart v3.1, 15 July 2008
  • Forum - http://opensolution.org/Quick.Cart/forum/
  • Demo - http://opensolution.org/Quick.Cart/demo/

Other e-commerce solutions

• VirtueMart - Mambo or Joomla! module
  • Website - http://virtuemart.net/
  • Latest release - VirtueMart 1.1.2, 31 July 2008
  • Forum - http://forum.virtuemart.net/
  • Forum members and posts - 113194 Posts and 29780 Members
  • Demo - VirteMart demo
• Ubercart! - Drupal e-commerce module
  • Website - http://www.ubercart.org/
  • Latest release - Ubercart 1.3, 30 July 2008
  • Forum -  http://www.ubercart.org/forum
  • Forum members and posts - more than 20000 Posts
  • Demo - http://livetest.ubercart.org/
• osCMax - ocCommerce fork
  • Website - http://www.oscmax.com/
  • Latest release - osCMax 2.0 RC3.0.1, January 2007
  • Forum - http://www.oscmax.com/forums/
  • Forum members and posts - 36912 Posts and 17924 Members
  • Demo - Demo at OpenSourceCMS
• WP e-Commerce - WordPress Cart plugin
  • Website - http://www.instinct.co.nz/e-commerce/
  • Latest release - WP e-Commerce 3.6.7, 8 Juny 2008
• CubeCart - not Open Source, but free
  • Website - http://www.cubecart.com/
  • Latest release - CubeCart v3 (free version)
  • Forum - http://forums.cubecart.com/
  • Forum members and posts - 125095 posts and 89,505 members
  • Demo - http://www.cubecart.com/demo/
• StoreSprite - not OpenSource, but free to use
  • Website - http://www.storesprite.com/
  • Latest release - StoreSprite v7 (needs registration for download)
  • Forum - http://forum.storesprite.com/
  • Forum members and posts - 2,892 posts and 296 members
  • Demo - StoreSprite demos