I'm getting really tired of dreaming up cool ideas for apps, cranking out prototypes, and then finding there is zero interest. Here are my four best ideas for what to work on next. I don't know how to "validate" them beyond a landing page/adwords. Last time I attempted that, adwords banned my campaign immediately.
LongCal: Basically, a calendar that goes beyond the standard month/week/day view. Emphasis on long term thinking and planning. I made a very rough prototype here (click on logo to get some test data): http://humbit.com/longcal/
uTrim: A URL shortener for universities. The idea is to create trustworthy links that can only be created by people actually associated with a school. I already have a functioning app and if you don't have an .edu account you can at least check out the landing page : https://utr.im/
Vulgat Library: Helps businesses create an internal lending library (movies, books, games). I sold this to two game companies, so it's the only thing I feel has proven demand. But I'd have no idea how to find more customers. This is the landing page that adwords objected to: http://vulgat.com/
Business naming app: After all the naming debacles I've seen on HN, I think an app that generates names (and checks against .com registration and trademark) could be really useful. My big doubt is if anyone would pay for it.
Do any of these sound worthwhile? Maybe I should just suck it up and do 30x500...
Make sure there is a large enough market before you put much more time in. Then, once you've got a concept that is workable and has a large enough potential market, try to convince your complainer-soon-to-be-customer to become your pilot partner. Then execute on the idea you have, make your first happy customer and then branch out to other people/companies with the same problem.