The Google Shopping Content Java SDK by Google aims to simplify API calls. An example shows hot to interact with Google Calendar, which can be useful to work with the Google Shopping Content API. Authentication Auth 2.0 intends to reduce the amount of code needed. See documentation available at https://github.com/google/google-api-java-client#Documentation
With Lemonade Stand, users can search for things to buy in a location-based manner, or add their own listings of things they wish to sell. The application is currently available as a client for Android, with a possible Windows Phone 7 version on the way soon as well, from a 3rd party developer that released an early version. Its Lemonade Stand API allows other developers to either code their own clients or use it in mashups.
Listrak is an email marketing company solely focused on the ecommerce channel. By getting to know their clients in depth, reviewing clickstream data and transactional history, and plotting customers on a lifecycle grid, Listrak is focused on delivering two things, according to CEO Ross Kramer, "help our customers acquire more new customers, and sell more to existing customers." The Listrak SOAP API makes automation of managing email campaigns easier.