* Recursive file system management tools. Most developers cannot navigate a tree model and cower in great fear

* performance analysis/monitoring tools. Most developers are astoundingly bad at performance and then lie about it with weird unfounded guesses

* markdown to html parser or the opposite. Most developers have no idea the actual cost of involvement required for basic string parsing and wrongly believe it’s free

* network proxy. Aside from authentication this is mind blowingly simple. Most developers will supremely over engineer this

* API documentation. In a world where the average developer struggles to write an email actually describing inputs, purpose, and outputs clear enough for a stranger to follow can be quite the impossibility

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That is taken from a recent comment in this thread: https://news.ycombinator.com/item?id=42157595