Ask HN: What to spend on to maximize developer utility?

For a couple of months it was https://www.saent.com/, hardware device that puts you (or apps) into distract-free mode. I stopped because communication (e.g. Slack) became too important to just block.

Also: second external monitor and both monitors having the same physical size and resolution.

Providing a free, substantial breakfast every morning. Gets people into the office way earlier too.

A library of programming books.