The long awaited Black Mesa Source mod is available to download (or at least it will be once the site comes back to life, it's just been hammered by thousands of eager fans). It's an almost complete (and completely free) recreation of the original Half-Life in the Source Engine that's been in development for the best part of a decade. It certainly looks like it exists, but then lots of things look like things, don't they? Clouds, for example. They often look like things.
The mod, in its current state, will feature Half-Life one content up to the Lambda Core, with the Xen bits to follow in a later version - one that the developers say will almost be a full game in its own right. The mod was also among the first projects to be Greenlit for a Steam release, though that version of Black Mesa will take some time to appear. Interesting bit of trivia, courtesy of @ultrabrilliant: "Black Mesa is launching at 8:48am New Mexico time, because that's when Gordon arrived for his first day at work."