http://www.adaptiveaccounting.com/

Found it...the basic package allows you to write macros in VBA, this isn't that much use, but can be handy.
The enterprise package on the other hand is slightly different:
For larger organisations, whose processes need to be backed up by bespoke systems, Accounting Office provides an environment which allows you to redesign any part of the front end. The Enterprise Development Toolkit (EDK), delivers the Visual Basic source code of the user interface in the form of a VB project.
The main problem here is "For large organisations"...Hehe.
You will only get the UI code, which can be useful, but this won't let you change the functionality of the application. This just means you can display data, to the user, in a different way.
One good thing, is that it uses SQL Server. This means that you can get a VB/C+ developer to write a bespoke program, which uses the data in the database...Now that is handy.
Oh, by the way, you will pay, I imagine, a damn site more for having the source code for the UI.

Regards,

Woka