I'd call it a mysterious countdown, but everyone had pretty much guessed what the Black Mesa site was ticking down to. That site has now tuck its last tick, and lo and behold here's Black Mesa on Steam.
Specifically, it's on Steam Early Access. That's because, as when the Half-Life remake was first released back in 2012, the Xen chapter still isn't finished.
"We currently have the first ~85% of our single player campaign completed," explains the Early Access Q&A. "For the last 15% we want to not only recreate it, but improve upon it to make it an enjoyable and memorable experience."
The Early Access version includes new visuals, new voice over, updated encounters and stability changes, and also adds multiplayer. The final release will feature more maps and modes than are currently included, but the presence of a Steam Workshop page for Black Mesa should ensure some additional arenas to tide fans over.
Black Mesa is currently available on Steam for £15/$20, and that price is set to rise when the game leaves Early Access. Given the recent furore over paid-for mods, I'll be interested to see the reaction to this—especially given that the download link for the previous (and free) mod version has disappeared from Black Mesa's official site, if not from ModDB.
Update: The Black Mesa website has been updated with a download option for the original mod version.