I couldn't resist and here are a couple of my favorites from the OBA eivdence on Microsoft Case Studies. Like friends, pets and children, all of the stories are great, but these, for one reason and another, piqued my excitement:

Customers

Banco de Venezuela
Using Visio to help visualize customer claims processe in real-time

Calypso Healthcare Solutions
One of several great BI-based OBAs that extends the benefit to legal teams beyond the firewall using Groove 

Canada Post
Canada's largest SAP customer facilitates data upload to SAP's applications using WinShuttle technologies

Energizer
A pioneer of Microsoft "online"

London Stock Exchange
One of the world's pre-eminent financial exchanges using OBAs to enable its people to improve customer service

Schneider Electric
An example of a WPF client being used in an OBA

Skanska USA Building
Many good pieces here but, in principle, a great way to derive value from a SOA implementation using OBAs

WorkflowOne
An Oracle customer that augments its Oracle ERP system with an OBA

Partners

Approva
A compelling ecosystem ISV that is reducing development time and helping its customers with its OBA

Convergence Communications
An OBA in-the-cloud that enables its event organizing customers to rapidly deploying its solution

Dassault Systèmes
An OBA helps revolutionize the process of managing fault diagnoses

Epicor
One of Microsoft's Global ISVs that's benefiting from wider use of its software thanks - in part - to OBAs