Some people responded to my "OBA/Office Mashup" post with some descriptions of OBAs that they had written. I'd love to get a link to some screenshots of an OBA you've built with VSTO and I'll add it as "OBA of the Day" to this blog.
I'll start this new recurring "OBA of the Day" feature with an OBA that was done by Infusion Development and blogged about by Kurt Guenther here and here. I love his quote in the PowerPoint deck from a customer who looks at the default Office "Getting Started" Actions pane and says "I hate you 'Getting Started'! You do nothing for me, GO AWAY" and then was amazed when the Actions pane was extended with VSTO to show more relevant actions for that persons job. That's kind of one of the major points of the whole OBA thing--contextual actions that are actually relevant to your job presented within the familiar Office environment.
Infusion customized Outlook 2007 for Elite Model Management and added a custom task pane that helped the bookers in the company more easily manage the calendaring and scheduling of models. You can see a screenshot of the OBA here:
The complete case study can be found here.