Here you can find a task list about changed made to Drupal in order to solve some xhtml or accessibility issues.

I'm using the Tasklist module that you can find at drupal.org.

Description of the task module:

This module enables a collaborative tasklist to be maintained. Tasks can be assigned to users, who can also colour code their tasks by editing their account settings. Tasks can also be reordered and when complete, a completion date set. Tasks with sub-tasks become tasklists themselves. There can be an infinite depth of tasks with sub-tasks, but only one 'master' (top-level) tasklist.