The pair of trailers are for a Half-Life short called "The Fall Down Of Evolution." Created by Turkish director and VFX artist Tiber Ergür, it will tell the tale of a young soldier as he tries to survive the Combine invasion of Earth during the Seven Hour War -- but for now, all we have is two really great teasers.
First up is the "Evolution" teaser. It is fairly low-key, and presents a more thoughtful, if sinister, view of the invasion:
The second one is kinda terrifying. Fittingly, it's the called the "Aggressive" teaser:
The progress chart on the film's official website shows that there's quite a long way to go until the finish line. Still, if you liked the trailers, you might wanna keep an eye on it for updates.
Half-Life: The Fall Down Of Evolution [Official Site]