It's unofficial, it's only the first (map) in a series of episodic releases, but it's fucking good. Well mapped, well entity'd, well written, well presented, challenging, fun, and seems to slot into the HL2 storyline/events fairly neatly in that it takes you to an alternate part of it, similarly to Opposing Force.
You play as a combine soldier -- kind of. I wrote some more about it and took some screenshots here and really must stress that it's at least worth trying out.