GNOME forks Cinnamon and Mate (active), libav vs ffmpeg (now closing), mplayer/2/mpv (mostly overtaken by last one).

Most common pattern is multiple packages solving the same problem, rather than forks - and incompatible versions.

The rest are good examples but of the millions of projects very very few are forked successfully

I could add a couple to this list as well but that does not change the fact that successful forks are exceeding rare