According to Valve, it's holding the promotion to help players get ready for Half-Life: Alyx. The upcoming VR title is set before the events of Half-Life 2 and will feature many of the characters that play a part in later titles. "The games share characters and story elements," the company said. "The Half-Life: Alyx team believes that the best way to enjoy the new game is to play through the old ones, especially Half-Life 2 and the episodes, so we want to make that as easy as possible."

If the now quaint graphics of the original Half-Life put you off, make sure to also check out Black Mesa. It's a fan-made recreation of the 1998 classic that adds new, more modern graphics, as well as tweaked gameplay elements, areas to explore and more.