http://www.bbc.co.uk/news/technology-35329209

Prospekt is made by 25-year-old indie developer Richard Seabrook, who over two years ago decided to quit his job and make the game as a job application.

Valve, the company behind Half-Life, have given Seabrook approval to use graphics and assets from Half-Life 2 and while the game is not an official Half-Life title it has been approved by the company.
It's not free and will cost several pounds/dollars...