ScummVM is a program which allows you to run certain classic graphical point-and-click adventure games, provided you already have their data files. The clever part about this: ScummVM just replaces the executables shipped with the game, allowing you to play them on systems for which they were never designed!
Originally it was designed to run LucasArts' SCUMM games, such as Maniac Mansion, Monkey Island, Day of the Tentacle or Sam and Max. SCUMM stands for 'Script Creation Utility for Maniac Mansion', which was the first game for which LucasArts designed this system. And much later it gave its name to ScummVM ('VM' meaning Virtual Machine).
Over time support for a lot of non-SCUMM games has been added, and ScummVM now also supports many of Sierra's AGI and SCI games (such as King's Quest 1-6, Space Quest 1-5, ...), Discworld 1 and 2, Simon the Sorcerer 1 and 2, Beneath A Steel Sky, Lure of the Temptress, Broken Sword I and II, Flight of the Amazon Queen, Gobliiins 1-3, The Legend of Kyrandia series, many of Humongous Entertainment's children's SCUMM games (including Freddi Fish and Putt Putt games) and many more. You can find a full list with details on which adventures are supported and how well on the compatibility page. ScummVM is continually improving, so check back often.