Every day, thousands and thousands of applications re being developed worldwide. Out of that pool, only a handful survives per month. As with all inventions, some of them have the potential but they don't make it due to different reasons, like bad marketing and such. Some applications are bound to survive though for a very simple reason : because they operate in a very niche market. Text4Deaf is one of those survivors. It won't die easily even if it is a bad application. As long as it works, people in that niche will use it, because it was written for them. But the good thing about text4deaf is that it is actually really good, and inexpensive.
Technology Lines has a good example of another niche - read the entertaining story about how someone came to develop a site where anyone can log on and buy a contract to protect themselves from unseasonable weather.

What great idea do you have ? Tell me in the comments.