I'm going to reply strictly from the top of my head because the deeds of a witness should be known without having to be looked up (I didn't read the replies here before typing this up). This is an incomplete list of achievements for that reason.

Here goes nothing: @emrebeyler built Dpoll, a way to add polling to steem. He's also got a lot of python contributions, the exact name of his repository escapes me but you can pull it up easy. @pjau built the steemcleaners blacklist api, spends a ton of time catching phishing and plagiarism, and works with the steemcleaners. He also built a guessing game. @therealwolf made Smartsteem and with it introduced a whitelist model to the community (I believe that's over now). He's also an active contributor on github and was instrumental resolving the post-hf situation with his cli tools/assistance. @c0ff33a is the partner of @derangedvisions, the both of them organizing quality engagement opportunities for new users while contributing to a whole list of projects.