What are "add-ins"?

Add-ins are programs that you can install and use with Outlook to "extend" Outlook's capabilities.

In Outlook 2007, add-ins are in the Trust Center, accessed though the Tools menu..

 

While many add-ins are under COM Add-ins, a few are listed under Exchange Client Extensions (the Add-in Manager in older versions)

 

In Outlook 2003 and older, the Add-ins dialog is at Tools, Options, Other tab, Advanced Options button.