Sitecore has a number of attractive eCommerce options that compliment Sitecore XP.  While the three solutions below are very different in what they offer, they're similar in that they all integrate directly with Sitecore. 

Sitecore Commerce

Sitecore Commerce is the next generation of the legacy Microsoft Commerce Server product.  Sitecore acquired the product in November of 2012 and is building its enterprise commerce offering around the product.  There are a lot of questions about the maturity of this solution and the product roadmap.  If you're considering a Sitecore based eCommerce platform, we can help fill in the gaps on this one. 

uCommerce

With Ucommerce you get the market's most powerful .NET based commerce platform, that is fully integrated to Sitecore.  The Commerce catalog foundation supports rich catalogs with products you set up to support multi-store management.

inRiver

inRiver is the market leader for simplifying Product Information Management (PIM).  The powerful inRiver PIM platform radically facilitates the creation, handling and distribution of perfect product information for a world-class customer experience in all channels, in multiple languages.

Is eCommerce part of your Sitecore roadmap, or are you considering Sitecore as part of an upcoming eCommerce project?  If so, feel free to reach out.