Sometimes your "killer" idea might be selfish or for a really small niche, too. It may not be a successful business venture, but it could be the killer app for you, friends and family, or specific types of users. For example, in another blog post of mind somebody older commented on his desire to have an app that simply uses the camera to automatically enlarge what he's looking at to help him read. And if you're interested in learning or building things then perhaps working on your own personal project will be super rewarding.