[Birdhouse][link], co-created by [Adam Lisagor][adam] of [You Look Nice Today][ylnt], is a great example of an application made for a niche audience, with the application’s authors being a part of that niche.
[adam]: http://lonelysandwich.com/ “Adam Lisagor / lonelysandwich”
[link]: http://birdhouseapp.com/ “Birdhouse — A notepad for Twitter”
[ylnt]: http://youlooknicetoday.com “You Look Nice Today”
Birdhouse is for twitter writing what [MarsEdit][mars] is to blog writing.
[mars]: http://www.red-sweater.com/marsedit/ “MarsEdit”