Project Insight has the ability to link to an unlimited number of external systems.   With the Link/Map feature built into Project Insight, a relationship between external records, web pages, or other data can be established and stored.     Once the relationship is established, items in Project Insight can be looked up using the ID from the external system.  For example, an Issue can be created in Project Insight using the API/Web Services (Web Services see DomainIntegrationObject), and the ID of the external system, in this case ZenDesk ticket number can be stored in a relationship with the Project Insight Issue.   When the Issue is displayed, a hyperlink to the ZenDesk system will be presented, so users can simply click the link to view the item.   The external system ID, in this case the ZenDesk ticket number can be used to lookup the Project Insight issue.   

The video shows how to set up the simple integration between ZenDesk and Project Insight issue.   
Online 2/10/2015