Though GOG recently branched out into selling new games too, it's still clearly committed to bringing gems back. Bullfrog's classic sim Theme Hospital is now on GOG at $5.99, bringing patients suffering from gigantic bloated heads or delusions of Elvis through your doors.
Theme Hospital's a fine example of Bullfrog's skill at making 'funny' games. While patients suffer from absurd illness, the treatments are ludicrous and vomit flies freely, it's all built upon a compelling management sim. Given an empty hospital, you have to design and build every room, arrange its contents, optimise your hospital's layout, hire staff, research treatments, and keep everyone content.
Theme Hospital hasn't been sold for many moons, so head on over to GOG with 599 shiny virtual pennies to buy it, you.
You may also want to try CorsixTH, an open-source replacement engine for Theme Hospital. It's not quite complete yet, but offers support for higher resolutions, and runs on Mac and Linux too. Simply plug your Theme Hospital data files into it and away you go.