Black Mesa, the extensive remake of Half-Life 1, is very close to completion. So close, in fact, that developer Crowbar Collective wants players to test the release candidate build.
In a Steam announcement, Crowbar Collective announced it has pushed a complete build to a public beta branch, with the intention of this build then becoming the same one released when the game comes out of early access. In effect, unless there are any major, unexpected issues that need fixing at the last second, this is the release build of Black Mesa.
However, Crowbar still wants people to test the game and push it to its limit. In the announcement, it says it needs people testing everything from crashes and achievement unlocking to pacing and difficulty spikes. While there is still a laundry list of known, minor problems—mostly UI errors—Crowbar expects to be able to release a full list of patch notes for the 1.0 launch very soon.
Black Mesa has been in development for a very, very long time. Starting life as two different mods for Half-Life 2 in the mid-2000s, it got the go-ahead from Valve to release as a standalone game in early access in 2015. At first, only the Black Mesa part of the game was available, with the later Xen levels still being in development until finally being fully added in December 2019. Since then, Crowbar has been polishing and tweaking the game, leading up to the imminent 1.0 launch.
An actual release date for Black Mesa hasn't been announced. But if you want to try out the full campaign as it may be at launch, go to your Steam library, right-click on Black Mesa, click "properties", then click "betas", then select "public-beta" in the dropdown menu.