In the same kind of vein as imagemagick, Dave Coffin's dcraw tool at least partly underlies almost every non-proprietary RAW image decoder, and some of the commercial ones (if they don't use code, they use constant matrices and such).

He's not a sole maintainer to any of his major projects anymore, but honorable mention to Fabrice Bellard who initiated both ffmpeg and qemu among other notable activities.

IIRC the Expat XML parser that's embedded everywhere was basically on spare-time maintenance by Clark Cooper and Fred Drake for a couple decades, but I think they have a little more resources now.

SQLite is a BDFL situation more than single-maintainer, but D. Richard Hipp still has his hands on everything, and there are only a relatively small number of folks with commit access.