Black Mesa has been officially released by the aptly named Black Mesa Modifcation team. The Source Engine upgraded version of Half-Life isn't yet available through Steam, but the developer's website has multiple options for download (it's just a little borked right now). The best part is, the modification is completely free! In order to play Black Mesa Steam will have to installed, as well as as the Source SDK Base 2007 which is also free. Everyone should be crackin' crabs with a crowbar in no time.
In development since 2004, Black Mesa's goals have been to create a modernized Half-Life built on the Source Engine that fans could be proud of. It won't be the first time Half-Life was ported to Source, Valve did that themselves with Half-Life Source in 2004. The Valve port was met with mild dismay that Half-Life hadn't been upgraded to the extent it could have been. At the time, even Gabe Newell acknowledged the port's faults, but insisted the fan community would inevitably create a superior product -- and how.
One of the most anticipated modifications in years, Black Mesa was quickly voted one of the first titles to be accepted through Steam's Greenlight system. Play the classic Half-Life, updated for a modern audience, right now. Nihilanth, the G-Man, and head-crabs small and Gonarch await.