Ingo Renner has written a nice article about extension keys, maintaining them and pass them to others. Here is an extract of that article right from <link http: www.ingo-renner.com>his blog.
Now what I'd basicly like to tell you with this is that if you feel that you can't support maintainance and/or development of your extensions anymore, go ahead and offer maintainership to others. The best place to do this are propably the mailing lists and newsgroups. You might want to try dev, english and ect or any other list that you think would fit.
It is important for us and the open source community in general that we don't start developing the wheel over and over again. If you just take a look at the number of calendars that exist in TER it becomes clear that a lot of work could have been saved if efforts would have been joined or development would have been taken over by others after an author figured that he hasn't the time to support his work anmore instead of starting over from ground up. If you think you found an extension that nearly does what you're looking for but it does not quite, don't go and rewrite it. Instead go ahead, make your changes and get involved in development by sending these changes (as a diff) to the original author and thus encouraging and supprting further development of that particular extension.
Back to transfering extension keys: If at some point in time you feel like not being able to support development of your extension just go and and offer it at one of the lists like mentioned before. There's another issue with all that: Some people like to put prefixes in front of their extension keys to better distinguish them from others, to advertise, to follow the hints for "good keys" in TER of for whatever other reason. To see what your extensions are you can just go to the key manager in TER where it will list what extensions are yours.
At this point I have to disagree with the very first hint at that page which suggests to put these prefixes in front of the keys. I actually also have to admit that I did this with Modern FAQ (EXT:irfaq), too. To encourage people to take over development of extensions I think it is motivating if the key has no prefix. For example if tt_address would have been named ks_address I think it would not have been as much motivating to take over its ownership for me as my initials are "ir". Choose your keys wisely and in a more general way like "calendar", "guestbook" or "bookreviews" for example to keep any opportunity like handing over ownership open to you and the others that come after you. If you want to use your extension keys to advertise, which I think many people like to do you instead can do this with things that can be changes afterwards like the extension's title, description or manual.