Chromium is a fork of Webkit which is a fork of KHTML. If you use a web browser, that browser is either Firefox or a fork (of a fork) of KHTML. I don't know what percentage of Chromium/Webkit users have heard of KHTML, but I'd recon it's on the order of 1%.

mplayer used to be a very popular, very good media player, mostly for linux but it was cross platform. Development sort of died. These days its fork mpv is much more common.

There used to be many different forks of GCC. In 1997, all of these developers merged their forks together into the EGCS project. This project proved to be very active and very...good. It was so good that the FSF halted development on mainline GCC, forked EGCS into the new mainline GCC, and restructured the community built around the ideas of the EGCS community. If you use GCC, this is the version you use; forked from GCC into EGCS, forked from EGCS back into GCC.

MacOS' kernel is a fork of FreeBSD. Much of its userspace is a fork from ... somewhere. If you use MacOS it's forks all the way down.

Kindle is forked from Android and/or linux and/or... well it's complicated. Amazon forked a lot of stuff.

Much of the Android ecosystem has been forked. OpenSSL was forked into Tink, for instance. Again, complicated, lots of forks.

yt-dlp was forked from youtube-dl when ... the lawyers came.

Ubuntu is a fork of Debian.