A countdown link on the Black Mesa mod website probably signals a final release date for the Source Engine-powered remake of Valve's original sci-fi shooter Half-life.
The first half of Black Mesa was released back in 2012, and technically the initial germ of the idea emerged back in 2004. Which is a very accurate recreation of the Valve release schedule, come to think of it.
Black Mesa Trailer 1 - Mod DB
Back in 2013 the mod team behind the project took Black Mesa to Steam Greenlight, where it was quickly rushed through by fans. Since then the volunteer team has been beavering away to get the mod ready to sell on Steam. There will be two versions, a free one and a paid for version, and the team says that "the Steam version will include features that the free version simply can not have". What exactly that means in terms of content, we don't yet know.
The countdown timer seems to point to May 5, so we'll check back then to sew what's going down. If you fancy running through the first half of Black Mesa (the first half of Half-life is clearly the best anyway, right?), you can grab it for free here.