Valve is going to publish Prospekt on Steam, a fan-made sequel to Half-Life: Opposing Forces. The game uses Half Life 2 assets, with Valve's permission, and offers 13 new levels with fully-scripted puzzles and action sequences.
Prospekt was made by 25-year old Richard Seabrook, he carefully integrated this stand-alone DLC into the Half-Life 2 story and updated the game with new textures, new models, new effects, new music, new voice acting, etc.
Prospekt begins in the Nova Prospekt prison in the Half-Life universe. Gordon Freeman is slowly being overrun by soldiers in the prison, however unknown to him his Vortigaunt allies manage to find some help from a forgotten hero.
You can find the game details over here and pre-order it via Steam for $8.99 (pricing will increase to $9.99 from February 11, 2016). Still no word about Half Life 3 unfortunately!