eBay4R is a Ruby wrapper for eBay's Web Services SOAP API. Has ease of use and small footprint. Instead of 40-50 line examples eBay gives you in C# and Java, with Ruby and eBay4R, examples are 5 lines or less.
Revel Systems API makes it possible to run a cash register, a restaurant and a mobile point-of-sale system with an iPad as the center of the user interface. Check out what Revel has to say about the API on their website.