Were it not for the core Half-Life series itself, Black Mesa:
Source - a fan-made remake of the original Half-Life - might
well be the longest-running tease in video games today.  Having
been in development for around eight years now, and still not
out, you'd be forgiven for thinking the project was never going
to come out.  That said, this clip - submitted to fansite
ValveTime by an "anonymous reader" - claims to show over three
minutes of gameplay footage from the mod, showcasing not only
its visuals, but a few changes as well, like an iron sight for
the .357 Magnum (which replaces the gun's standard "zoom").
Nobody has any idea how old the footage is, so for all we know
it could be from 2008. Or 2012. Or the distant future, where
humans toil for our alien overlords in vast underground salt
mines, and Black Mesa: Source still isn't out.