Like Half Life? Love Black Mesa

Via Kotaku

The long-awaited Black Mesa mod, based on the 2007 Source engine found within The Orange Box, has been completed and is now available to download. The mod is much more than a version of Half Life HD: Updated voice acting, levels, models and all sorts of other things that bring the first game of the series to the current generation.
And, best of all, it’s free!

Original Source: Kotaku: You Can Play Black Mesa Right Now; GameFront: Black Mesa is Live, Download it Here!