A teaser countdown on a website related to Half-Life fan remake Black Mesa has come to an end, and it's revealed a Steam Early Access version of the project that's now available. $20 gets you the Early Access version of Black Mesa, which contains roughly 85 percent of the single-player campaign. 

This represents an estimated 10 hours of gameplay right now--everything up to the Lambda Core chapter. That means the Xen chunks of Half-Life still aren't here; they were also missing from the free, unfinished version of the game released in 2012. This Early Access release also offers multiplayer, with deathmatch and team deathmatch modes on six "completely reimagined" maps from the original Half-Life currently available.

Black Mesa is a re-envisioning of Valve Software's seminal classic Half-Life. Gordon Freeman and his unforgettable journey through the Black Mesa Research Facility have been lovingly re-created to capture the full spirit of the original game, without 1998’s graphical limitations. This is Half-Life the way it was meant to be played! 

Expect tremendously detailed environments, old-school tough-as-nails combat, and a gripping story with memorable characters. The all-new soundtrack, voice acting, choreography and dialogue create a more expansive and immersive experience than ever before. Nostalgia has never felt so fresh! 

The Steam release of Black Mesa includes various fixes, upgrades, and new features since Black Mesa’s mod release. 

Heavily updated single player experience – The Black Mesa single player experience has greatly improved from the mod release; new visuals, new voice over, updated gameplay encounters, stability changes and more. Xen is not part of the Steam release of Black Mesa, but will included as a free update when it is ready. 
Black Mesa Multiplayer – Fight with or against your friends, in two game modes across six iconic maps from the Half-Life Deathmatch universe: Bounce, Gasworks, Lambdabunker, Stalkyard, Subtransit and Undertow. 
Custom Modding Tools – Use the same tools the developers used to create your own mods, modes and maps for Black Mesa and Black Mesa Multiplayer … 
Fully integrated Workshop – … and then share your work on the fully integrated workshop! 
Complete Steam Integration – Collect trading cards, backgrounds, emoticons, achievements, and everything else you would expect out of a Steam integrated game

More info here.