The best piece of software I’ve written is not interesting at all. It is dead boring, in fact. But it made a difference. Back in 2013, I made the Ruby gem HTTParty use OpenSSL’s VERIFY_PEER by default.
This requires the HTTP client the gem provides to verify the server’s certificate. Without it, SSL/TLS is subject to man-in-the-middle.
The reason I think it’s important is that the gem has the most use of any software I’ve written, with over 40m downloads. I think about 5–10 of those happened before my code was written, so I’ve got about 30m downloads plus of verifying peers.
And then there’s this :)