If you haven’t checked out Black Mesa yet, then you just aren’t a Half-Life fan. This fan-made Half-Life 1 remake is the recipient of numerous accounts of critical acclaim. It improves on the original Half-Life formula in every way: changing environments, adding new physics capabilities, and even including stages that were taken out of the original release. Valve has recognized the incredible work that went into the community-made project, and has allowed the game to be sold on steam.
According to the Black Mesa team, the game will eventually be made available on the Steam store for a “relatively low price,” though an official price wasn’t given. This paid version will have features that “the free version simply cannot have,” which is good considering you’d be hard pressed to get people to spend money on a product that has basically been free for years. The team also plans to open source their game maps and assets for the modding community.
Source: Black Mesa Steam Community