Modder ‘MrGnang’ has been working on a total conversion mod for the first Half Life game for five years. Half-Life: Echoes will let you experience the Black Mesa incident from a different perspective, with some insights into mysterious events which Gordon Freeman was not present for, and is available for download right now.
Contrary to other mods, this isn’t a standalone total conversion mod. What this basically means is that you’ll need a copy of Half-Life on Steam in order to play it. Also note that Half-Life Source and the non-Steam/retail version of Half Life are not compatible with it.
Those interested can download Half-Life: Echoes from here, and below you can find a trailer and some screenshots.