To celebrate the game’s 20th anniversary, Bethesda have released a free-to-play browser version of Wolfenstein 3D, widely considered to be the game that first popularized the first-person shooter genre. You can access the game here. The browser version contains all three of the original episodes.
Additionally, id Software co-founder, John Carmack, has released a developer diary of sorts, where he discusses the development of the game while playing through a portion of this. You can watch the developer video above.