Large subreddit where natives answer 'is this sentence correct?' within minutes.
Details
The most active free place to get a specific question answered — why one preposition and not another, whether a sentence sounds natural, what a piece of slang means — usually with several native speakers weighing in and disagreeing productively about register and dialect. Also a good barometer of which resources learners actually stick with. Answers come from ordinary speakers rather than teachers, so explanations of why can be wrong even when the correction is right, and it works best as a supplement to structured study.