Ask HN: What are some great resources you know of?

Do you know any great learning resources, interesting website/link, informative/interesting article or anything that can be useful or informative for someone, share it with a little description of it. Or a site you can waste your whole time exploring it. Thanks... For me it is Stumbled[1] (not StumbleUpon, discover interesting and awesome websites on internet) nowadays.

[1] Stumbled.to