Alan Kay is best known for his early work on object-oriented programming using the Smalltalk language which resulted in his Dynabook concept. His concept has never been fully implemented although a modern tablet PC running Squeak comes close. He is one of the core team that produced Squeak and he has also created another Smalltalk based project in 3 dimensions, Croquet. He has also done work in constructionist education.
This book chapter on Personal Dynamic Media shows what Alan was working on in 1977.
In 1987, while at Apple, he created a video called Knowledge Navigator to demonstrate his vision of fully interactive computing.
More info here: http://en.wikipedia.org/wiki/Alan_Kay
Partial transcription of Alan Kay's talk on the 40th Anniversary of the Dynabook Idea at the Computer History Museum: "after 40 years the dynabook is not here yet".