Well, it's not OSS, and it may or may not be as full featured as you require, but it might fit a lot of your requirements and I think the Interfaces might be free to use. Intuit's products have excellent User Interfaces, are widely used and are integratable with other products.

Intuit gives out SDKs and even has a Perl API for QuickBase, the back end database used by it's products.

Start here.