I have a personal theory that Reddit is a great source of startup ideas. There are many communities that have special behaviours that can be a source of inspiration for such ideas. A good example is /r/borrow where you can take a loan from another person in the community. But targeting niche subreddits is a very good strategy :)

/r/AskHistorians is an amazing resource.

SMEs engaged in oft-vigorous debate, accompanied by authoritarian moderation (to include sourcing requirements) generates some truly excellent content.