Let a user cancel his own account

It is a very huge topic.

http://drupal.org/node/8

There is a legal issue behind this and soon the opportunity for drupal to win more credibility and bring more people to use it by becoming a certified user management solution. (see for example http://www.truste.org/)

There are already two modules on this topic :
http://drupal.org/project/user_cancellation
http://drupal.org/project/user_delete

See also other threads related to this :
http://drupal.org/node/204495