Neat how involved he still is this long after the initial work.
When brotli came out he implemented his own decoder (https://github.com/madler/brotli/) to check the spec, and got some things clarified in RFC. Brotli is pretty DEFLATE-y in spirit, with changes that take advantage of today's hardware (larger history window, contexts in the entropy encoding) and a bunch of tweaks (different match encoding, static dictionary, etc.).
On the other hand he isn't making new zlib versions. There are a lot of patches posted on the mailing list going nowhere. Also there's a valgrind issue I reported that he ignored.
When brotli came out he implemented his own decoder (https://github.com/madler/brotli/) to check the spec, and got some things clarified in RFC. Brotli is pretty DEFLATE-y in spirit, with changes that take advantage of today's hardware (larger history window, contexts in the entropy encoding) and a bunch of tweaks (different match encoding, static dictionary, etc.).
Also, as others mention, NASA mission engineer: https://en.wikipedia.org/wiki/Mark_Adler#Mars_exploration