As you know, the Activity Manager (if you allow it to) is collecting the data about which documents you work on. The documents get automatically scored based on how often you open them, and how recent was the last time you did (it is a bit more intricate than that, but this is the general gist of it).
This has been used in Kicker to show the improved recent documents, contacts and applications (yes, contacts and applications are treated as documents) as well as in the Tasks applet and Plasma Media Center.
Apart from these few places, it is not used much.
One of the ideas I’ve had for some time now, but haven’t had the time to implement, is an advanced history browser.
The idea is to have the most popular documents (of a specific activity) shown in a timeline to represent when the documents were used the most. Also, it will be able to show other documents related to these.
So much for the time being, I’ll write more details when the concepts for the UI become more crystallized.