To uninstall an App Exchange App, perform the following steps:
 
1.)  From Setup, Navigate to Installed Packages.
2.)  Click uninstall next to the respective application you would like to remove.
3.)  At the bottom of the page, click "Yes, I want to uninstall this package and permanently delete all associated components"
 
If packaged components are referenced within Salesforce, you might receive an error message stating that the package could not be removed.
 
The most likely cause is that you have packaged components on layouts.  The error messages indicate which layouts need to be modified. 
 
For example, you may see:
 
"The object is in use by another object and cannot be deleted. ListButtons"
 
This means that the button is on the List View layout.  In this case, navigate to 'Case Search Layouts' and select edit next to Cases List View, and then remove the problematic button under 'Custom Buttons'.
 
Another example is an error saying:
 
"The object is in use by another object and cannot be deleted. Incident Layout"
 
This means that you should remove the button from the Incident Layout.  To determine which object the layout is related to, click on the button name in the Name column of the error page and then select the 'Object Name' field.  The button may be on a page layout for this object or a related list layout on a related object. 
 
Once you have removed buttons from all layouts and removed any other package references, you should be able to uninstall the package.

"The custom object is summarized in your organization."

This usually means that the fields referenced in the error need to be deleted. To delete them, find the Object they're in and do so from there. Then go into the Deleted Fields page and hard delete them. After that, go to the Recycle Bin and empty it, to make sure everything is deleted as expected.
 
For further reference on uninstalling app packages in Salesforce, please take a look at this Salesforce Success Community Article: https://help.salesforce.com/HTViewHelpDoc?id=distribution_uninstalling_packages.htm&language=en_US