I blogged briefly about the VSTA product before. VSTA is a new product from the same team that brought you VSTO. It takes the VSTO application customization technologies and makes them available for ISV's to license and integrate into their own application. Want to allow your users to extend and create add-ins for your application? VSTA is just the ticket.
There is a VSTA blog that you can check out for more information. Also posted there are two PDC sessions on VSTA technologies.