We're looking for an extension that will allow a user to enter data in fields within an article. this data would be unique to each user and accessible only to this user. Examples would be a "Mark as read" checkbox and personal notes for a piece of content. Does such an extension exist? thanks.

So you are basically trying to add CRM-like functionality, right?
I think if there isn't an extension posted at the JED, you should look for a custom solution.

It doesn't have to be a full CRM capability, but at least some ability for a user to add some personal data to a K2 resource that's unique and accessible only to that user. I have looked through the JED and didn't see anything that would do this. I was hoping if such an extension did exist, someone here in the forum would know about it. Anyone?