Shopping cart APIs, could you detail a bit what you're looking for?
Currently it is not possible to update the shopping cart of a customer using the API unless you know their entity ID. So for non logged in users, which is like 95% of the users shopping on your site, you can't do anything with the shopping cart (their order) until you find out their email address, which is basically the last step in the order process. There is no way to auto add things to carts, change details on the order, auto fill information into the order, or add multiples of products etc... which all would be great add on features.
Obviously the standard BC system is doing it via the order ID only which I would love to be able to do, update the shopping cart (order table) using only the order ID.