Watching The Core's trailer makes you forget you're looking at a mod for a game that came out in 1998. It's just gorgeous.
The Core is an in-development total conversion for the first Half-Life. It takes place during the resonance cascade, which is when Gordon Freeman and his scientist buddies accidentally unleashed the aliens of Xen on the Black Mesa Research Facility. Like in Opposing Force and Blue Shift, the two official expansion packs, you get to experience the cataclysm from a different perspective to Gordon's — this time, that of engineer Maurice Andrews.
The mod's been in development since 2008, so hopefully it should pop up soon. Here's hoping; I'd love to get my hands on this one. Check out the trailer below: The Core - Official Trailer Release [ModDB]