Allow users to annotate content in Drupal 7
The book Pro Drupal Development is probably the most popular book for learning how to build Drupal modules. In the 3rd edition, which covers Drupal 7, the author uses the Annotate module as an example of how to build a module from scratch.

If you read the first chapters of the book you could easily write the module yourself, or even better, try to port the module you mention to 7.x. By the way, remember to check Errata of the book first.