http://www.techradar.com/news/gaming/half-life-3-release-date-news-and-rumours-1290663

On 10th October 2007, Valve released Half Life 2: Episode 2, the continuation of Gordon Freeman’s story as he and his friends fought to free the human race from the oppression of the alien Combine. The ending provided very little in the way of closure, setting up a number of plot points for a sequel that has never arrived. 

With the exception of 2011’s Portal 2, the Half Life universe has remained completely dormant for almost ten years, which is a shame when Half-Life and Half-Life 2 are generally considered to be two of the best games every made.