The Hiveage PHP SDK by Lance Pioch interacts with the API to access billing features. Installation and usage available. Hiveage aims to help freelancers in over 140 countries to facilitate invoicing processes with their clients.
Every day ProgrammableWeb has a Mashup of the Day featured on the home page and the Mashup Dashboard. Today's is a unique one in that there's a lot more money at stake than usual: it's the Salesforce for Google AdWords application which combines Salesforce.com and Google AdWords.
Waiters like to work in pricey restaurants serving wealthy patrons because they are able to pay for high class service. That's also why you see so many professionals in the banking industry: it pays to work for people with money. Xignite took note of this and decided to become the Maitre'd of information to the financial industry. With more than 3.5 Billion calls per month to its Xignite APIs, it is doing a fine job of it, as well.