Last updated January 5, 2012.
If you are no longer capable of maintaining your project, please set the proper status for the project, add a note to your project page, and ask in the forums whether someone is willing to take over maintenance. The development mailing list is another good place to ask for volunteers. Proper communication is key so make sure to mark your project as orphaned.
If you are unable to find anyone to take it over and wish to completely disassociate yourself from the project, you can hand it over to the Abandoned Modules placeholder user. If you don't have access to do this yourself, file an issue in the webmaster's queue.
If you are willing to maintain an orphaned project, you should read the full project application process. Existing vetted users should submit a task to the Drupal.org webmasters issue queue to request an ownership transfer (there is an issue component called "Project ownership" you should use when submitting the task). Users who do not currently have permission to create full projects should then apply for full project permission making sure to mention the project you wish to take over in the issue.
For more information on Abandoned Projects check here: http://drupal.org/node/251466