Last updated 29 July 2013.
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 Unsupported Projects 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 Dealing with unsupported (abandoned) projects instead.