Add-on Per-user user notes

I've been looking for an add-on that allows any user to make notes on any other user -- these notes should be completely private and unique to users.

For example, John makes notes on users A, B, and C -- only he can see those notes.

Priscilla might make notes on A, B, and C that are completely different from John's and only she can see her notes. She cannot see John's.

Notes should be accessible in summary via mouseover and in full from the target user's profile (in a tab).

I'm actually pretty surprised something like this doesn't exist yet...