Valve has been teasing us with their Half-Life 2 themed Source Engine tech demo for some months now, and it has finally arrived. If you load up Steam you'll see it is available to download (at no cost). The Lost Coast is designed to demonstrate the advanced capabilities of the Source Engine, in particular showing off the use of HDR lighting.
It's worth pointing out that anyone expecting an expansion of sorts will be disappointed, as the level is very short, taking approximately fifteen minutes to run through. More Half-Life 2 will be available shortly, though, with the release of Half-Life 2: Aftermath scheduled for the tail end of this year. Aftermath is a proper expansion, offering a continuation of the story in Half-Life 2. picking up where the game ended.