A 13-level campaign, approx as long as Episode One, developed by one fan developer. Approved by Valve and passed by Greenlight.
A sequel of Opposing Force, taking place in Nova Prospekt.
http://www.geek.com/games/prospekt-n...steam-1644860/
http://steamcommunity.com/games/399120/announcements/

But seems a bit secondary to me, and also like a forced marketing.