IronStar,
Steve posted an idea that was useful to me, and that is to shift your thoughts from "how can I make the most money?" to "how can I create the most value to the most people?" Ah, here it is, number 3 in the article
5 Wealth Lessons from 20 Percent of a Millionaire. It's also number 9 in his article
10 Stupid Mistakes Made by the Newly Self-Employed.
Right now I am working on creating a flashcards website for early education. I came up with this idea because I wanted to make flashcards for my children and realized that if I was going to put that much work into something I wanted to do it online and share it with the world. I know it will be of value to me and my kids (my 2-year-old loves them), and I am inspired by the idea that it will provide value to others. I believe that because of its potential to create value that it will eventually be profitable, but I am still getting the structure in place and cannot say for sure. I also know that the PHP/MySQL skills I am developing can be of value to many individuals and companies, and I have discovered that I love building web-based applications.
So I think when you are evaluating an idea for its potential, your criteria should be (1) Does this inspire and motivate me? Is this something that I would love to do? and (2) Will this create significant value to others? If the idea passes those two tests, give it a try. You might fail, but you will learn something from the failure and your next attempt will be better.
Good luck!