The mega-ambitious, long-in-development Half-Life remake/tribute Black Mesa is finished, and you can downlaod it from Gamefront right now and give it a whirl.
You'll have to follow the instructions on the Gamefront page, as Black Mesa requires that you install the 2007 Source software development kit.
In the video above, you can see a comparison between the original Half-Life and the remake. Lookin' pretty good, Black Mesa. Let's see how you play.
Black Mesa is Live, Download it Here! [Gamefront]