Valves has given Half-Life fan remake, Black Mesa, permission to sell the game as a retail product on Steam.
Black Mesa is a re-envision of the original Half-Life. It was put together by fans over the course of several years. The remake was greenlit last year through Steam Greenlight program.
The team will keep the free GreenLight version of the game alongside with the newly approved paid version. Both versions will be more or less identical, whereas the purchasing the paid version is meant only as a way to support the team.