I think OXID CE’s biggest competitor is OSCMax2 (http://www.OSCMax.com )
I like all the features that OSCMax has for the US Market and how it handles Paypal intergration and the payment system (No Alertpay support yet) but it lacks a good front-end design like OXID CE is closer to. OSCMax’s front end is just too small, and sometimes you upload a picture to a product and it doesn’t show up in the product information. I think the two carts should merge forces and create an ultimate project. Both carts need better looking templates for the front-end design, and it would be brilliant to even include some Adobe Flash support and flash menus. Any method that it is done, both carts need better front-end design templates. It would be the perfect solution.