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Black Mesa is a mod for the Source Engine that recreates Half-Life 1. It has been in development since the release of Half-Life 2 in 2004, and has had its first release, which runs from the starting chapter Black Mesa Inbound to the climactic Lambda Core chapter, in September 2012. The final four chapters, constituting the Xen part of the game, and a multiplayer component are still under development and will be released at a later point.The time put into developing the mod certainly shows. Each level is updated and expanded, characters behave much more realistically, and the famous 4-scientist problem has been eliminated through the use of procedurally-generated faces. Add to this the fact that it's entirely free, and you really have no excuse not to give it a go.
Black Mesa provides examples of: