Black Mesa's ambitious expansion to Half-Life has reached the last stretch. After a technical beta in June, which tested a few maps, Black Mesa: Xen's first two chapters are now available in the public beta branch. Xen and Gonarch's Layer contain nine maps, so it's triple the size.
The other chapters will be added to the branch later, letting you test all 18 of the expansion's maps. Before you dive in, you can check out the known issues here. Developer Crowbar Collective says things will get quieter on the update front, but only because the team the expansion ready for launch.
Xen has grown over the years from a rework of the disappointing Half-Life level to a full campaign with lots of different environments and improvements. It was originally slated for a 2017 launch, but it's changed quite a bit from the original plans.