Recently I’ve been using AnswerThePublic.com every day. You enter any topic and it comes back with a web of search phrases that people ask about the topic.
I came across it on an SEO blog somewhere, and it has proved to be extremely useful. It really helps with writing more organized content. Using the questions that people ask as headlines helps SEO because it places the keywords in your headers. This nearly halves the time it takes to write a post, which is great!