One of the original games of exploration, Atari’s Adventure is now available online as a Flash Program.
Adventure defined a genre as the first graphic adventure game. It set the expectations and guided the development of everything that came after from Zelda to UT2002.
This game was one of the first loaded with hidden passages and easter eggs (hidden commands or game elements that are cool but have nothing to do with the storyline).
You’ll find it here: simmphonic.com. The reproduction is 99% faithful although Pehnke did move a couple of items around.