Akalabeth: World of Doom was Richard Garriott's first game, created as a school project back in the 1970s. It's famous mainly as the precursor to the hugely successful Ultima RPG series, although some fans go so far as to consider it the first Ultima game. Either way, the point that's of interest to us today is that it's free for the holidays on GOG.
The GOG package includes the manual, a wallpaper, and Windows, Mac, and Linux versions of the game itself. It's primitive stuff, even by retro-gaming standards, but did I mention that it's free? For those with (slightly) more modern tastes, the Ultima series is on sale as well: Individual games or bundles (including the Underworlds) are going for just over $1, or you can pick up everything for $6. The sale is on until December 26.