It's free to all HL2 owners, and promises about 15 minutes of new gameplay from levels that were cut from the game before release, some hot HDR action, and apparently, commentary from the game's creators. Restart Steam and get downloading, fools!

EDIT: Okay, just finished playing Lost Coast, twice.

The level, in terms of gameplay, is very short, no more than a few minutes worth of action. Still, it's fun, and it's easy to spend long minutes exploring and gawking at the sights and the HDR effects.

I also played it with the audio commentary on. Basically, there are little floating icons you can click on as you progress through the level, allowing you to hear various Valve team members talk about different aspects of the level. Highly enjoyable and interesting, and it's more than just audio in some cases, where you are actually shown things on your screen.

At the end of the level, Gabe Newell invites feedback on the commentary: if you enjoyed it, let them know, and they'll incorporate it into future projects.

Personally, I'm an audio commentary junkie. I love DVDs with commentary (which is why I have this Notes section, to do my own half-assed commentary), and the prospect of PC games with commentary, I think, is an exciting one. If you do enjoy the commentary on Lost Coast, I urge you to send e-mail to Gabe to let him know, because I'd love to see this feature implemented in the future.