There are many varieties of open source shopping carts are there which are generally simple to integrate with your existing sites and as well easy to customize, the only question is- which would be the best? I would suggest before choosing shopping cart software either its free or paid, you should examine issues of installation part and general effectiveness and make sure the ecommerce software you are going to choose should meet your customers need autonomously. Check your shopping cart should follow the latest technology for example it should easily integrate with popular payment gateways including Google checkout system which is becoming popular too.

Try to find user reviews either it is free or paid service through forums. In general many features of well known shopping cart software are intuitively designed for business users but you should also check it should be search engine friendly and must be security built up. Especially for paid service check the support and level of customer services of the providers by raising few pre-sale questions and check the response time and the quality of the reply. Check thoroughly the manual and support documentation, download the user guide and take a close overview before you buy the shopping cart software.