Over the past few weeks, I've described the steps involved in creating an Outlook add-in, including ribbon manipulation and access to the Outlook object model. To wrap up, the table of contents for the VSTO add-in project:

There are many other examples of the use of VSTO, as a quick web search will tell you...

The VSTO MSDN entry page is http://msdn.microsoft.com/en-us/office/hh133430.aspx and http://msdn.microsoft.com/en-gb/office/hh128768.aspx talks about Outlook programming.