Quora is probably one of the better resources on that list because rather that listing your product on the shelf next to 1000 others, it allows you to discover people looking for solutions just like yours.

For example, say you're building an Android weather widget (for some reason). Build a profile where you're an 'Android', 'Widget', 'Weather' expert and Quora will surface this question for you to answer* : https://www.quora.com/Android-Application-Which-is-the-best-...

This targeted approach is much more valuable for your product, your time, and for the person asking the question.

*Of course it helps to actually have the best weather widget.