The Internet Archive exists to preserve a library of digital content -- just as a brick-and-mortar library or a museum preserves physical items -- that are culturally significant. And games -- of course -- fall into that category. Ask any gamer what was the first game they played, or the first game they fell in love with, and sit back and prepare for the passion.
Now gamers older and younger alike can access some of the most iconic older MS-DOS titles for free, thanks to the Internet Archive's efforts (2015 is off to a productive start). At time of writing, the organisation had collected some 2,314 old MS-DOS games -- games that are no longer playable on current platforms, and therefore might be considered abandonware -- and made them available for free on its website.
Titles include 1990's Prince of Persia, 1990's The Oregon Trail, 1997's Bust-A-Move, 1992's Wolfenstein 3D, the original Metal Gear from 1990, 1987's Maniac Mansion, 1989's Sim City and even a fan-made update of 1986's original The Legend of Zelda.
The games are playable using a browser-based emulator created by the Internet Archive for this specific purpose, EM-DOSBOX. It's still in beta, which means it may occasionally experience a bug here and there; and, of course, there are no manuals accompanying the titles, so you may have to rely on trial and error to figure out how to play.