Drupal-yellow-marker is a project mainly written in ..., it's free.
A drupal feature based on jQuery Text Annotator
This project is a collaboration around http://pkedu.fbt.wur.nl/~koos/jQueryTextAnnotator/ where Koos is trying to integratie with Drupal.
The first goal is to grasp features as in "is this possible with features"
When a stable module exists the module will probably get part of http://drupal.org/project/annotate which is related to but not as cool as this yellow marker stuff aka text annotator.
As features are depending on modules here a short list of modules needed.