The Fool and His Money.
Popular puzzle game, The Fool's Errand is released in 1987.
Sequel announced with expected release in 2003.
Sequel actually comes out in 2012.
I remember when it came out, everyone who remembered it was shocked that it actually existed.
I've got some Kickstarters that, while not quite DNF levels, are well on their way.
Heart Forth, Alicia
Kickstarter funded (so they had already been working on it before this) - May 2014
Estimated delivery - April 2015
Still not released and they haven't committed to a 2020 release date yet
The developer releases frequent updates with gifs. I played their demo at E3 a few years ago and it was rather good. I imagine the game will come out at some point and isn't vaporware. But yeah, it's taking a LONG time.