The new epitome of vapourware is....
GNU Hurd is a replacement GPL OS kernel that would be like Linux, only apparently much better. In development since 1984, still doesn't work (as far as I can tell).
Project Xanadu is the original Hypertext project from 1960, predating the web by aaaages. The web is a pale shadow of the sheer awesomeness Project Xanadu would bring to the world. Sadly, a complete implementation has never been released.
WinFS is the object oriented filesystem for Windows, allowing applications to store their data in a more structured manner and bringing all sorts of amazing possibilities to your user experience. First announced as part of Cairo, Microsoft's OS of the future, way back in 1991. Still not quite here.
Elite 4 is the successor to the legendary space trading/shooting game I played back in 1984. Mentioned in hushed whispers since 2006.
Half Life 2: Episode 3 follows on from the Legen...wait for it...dary Half Life and HL2. Part of Valve's foray into episodic gaming, which would allow them to get their games out to the public really, really quickly, with none of the tedious waiting around that happens when whole games have to be produced. Part one appeared in 2006, and part two came out in 2007, so presumably part three came out three years ago now.
*Software that's been announced but never been released.