The original Fallout? But it won't run on Windows 7 (or even XP as I remember). Still AWESOME game.
lol. I swear, every time there's a thread about GOG.com, there's always a newb who says "But these game(s) are really old and won't run on newer computers!"
Actually it will. That's the entire point of GOG. They take old, classic games, pre-mod them to be 100% compatible with Windows XP/7, and sell them for cheap. The game should install and run out-of-the-box with no issues whatsoever.
HOWEVER, there are a bunch of unofficial patches that I recommend installing for a better experience. They fix a lot of in-game bugs, improve the graphics, allow you to play in widescreen resolutions, etc, etc:
http://www.gog.com/en/forum/fallout_series/fallout_mods/page1