Grim Fandango
In addition to being a very good adventure game, it features great writing and beautiful art direction.

The Oregon Trail
The Oregon Trail is an edutainment computer game about American pioneer life that has a long history in North American school districts and homes. The game was inspired by the real-life Oregon Trail and was designed to teach schoolchildren about the realities of 19th century pioneer life on the trail. The player takes the role of a party of people, of a selected profession, traveling across the Oregon Trail in a Conestoga wagon.

Sanitarium
Broken into eight chapters, the entire game is a journey through your own madness to potential salvation.