Ask HN: Which market testing tools have you found useful?

I admit it: I've made the typical developer mistake more than once of building a product that I thought was "cool, and easy to whip together with Rails." I've said things like, "I'm sure that we'll be able to find users for it after we can show it to people."

I'm finally over that mentality, and from now on I will only build products after collecting thorough data that demonstrates a market need. So, I'm wondering what tools hackers here have found especially useful for finding markets and testing viability of digital products (web services, apps).