freshbooksjs is a community supported Node.js wrapper for the Fresh Books API, an online invoicing and time tracking service. Instructions on how to implement the wrapper are documented on GitHub. The package is under MIT license and is maintained by Sabra Pratt.
PlanetRe, a cloud software vendor for real estate, has integrated Intuit's Quickbooks into its planetRE Money platform using Intuit's API for the popular accounting software. Intuit's API is among 46 accounting-related APIs in our API directory. Intuit's RESTful API is part of its Partner Platform, whose documentation covers everything from pricing to the development lifecycle.
FinancialForce.com, focused on building cloud based business applications on the Force.com platform, has released an API for its accounting application (i.e. The FinancialForce Accounting API). The API gives access to the entire suite of accounting tools available through the accounting platform (e.g. cash management, reporting and dashboards, ordering and billing, etc.). Although FinancialForce.com built the application with Force.com, and the target audience began with Force.com developers, the application has been integrated with many platforms and APIs (e.g. Google Apps, Basecamp, Facebook, PayPal, etc.)