Source: Half-Life Fallout

Bite from Half-Life Fallout has put online a brand new Half-Life 2 story speculation article. He used a lot of the recently released media pieces (The Bink movies, the Prima Games book previews etc) and used them to for a possible story that leads up to the events of Half-Life 2. Do note that the article does contain a lot of possible spoilers, but here's a preview that does not (At least if you have played Half-Life):

The resonance cascade that occured at Black Mesa teleported a number of alien lifeforms into Earth. Now we know that the Test Chamber served as the centre of this vast teleportation event, and slowly sent out 'ripples' which expanded upon the teleport events. While you were battling aliens (and marines), these ripples continued to spread outwards, slowly engulfing the whole of the Earth. Black Mesa was eventually destroyed, but the damage had been done. Xen aliens had invaded the world.

The full story, which is a great read, can be found here at Half-Life Fallout.