You can integrate any system that provides web services or an API or consumes web services because CRM provides a rich SDK that includes the use of Jscript for client side scripting, embedded Silverlight apps, Iframes to expose other .NET apps,
and also .NET integration through plugin and workflow assemblies.
So there are a number of options. I recommend looking through the code samples in the SDK and also looking through the included .CHM file that contains all the documentation.
Here is a link to the SDK:
http://www.microsoft.com/download/en/details.aspx?id=24004
Jamie Miley
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