Have you ever found yourself saying "I wish there was someone that did [insert]"? You then scramble around, ask a few people, put out a few inquiries, perhaps run a google search, and low and behold, nothing. Without thinking much about it, you just stumbled upon an untapped opportunity. You've identified there was a need (because you just wanted it), and you've also, in a quick way, demonstrated that there was a lack of providers for that need. But that's often where the thought process stops. You might lament the fact that nobody is doing this thing that you've identified a need for, perhaps vent your frustration in a blog article or twitter post that you can't find this/that close to you, and then move on, perhaps missing out on a golden opportunity.