Anyone who works in interactive computing should be familiar with Doug Engelbart's work.
Years ago, at one of the first conferences on interactive computer, after people spent all day presenting their new work, Andy van Dam--builder of the first hypertext system with Ted Nelson who named it--stood up and said "you should all be ashamed that you don't know your history. Doug Engelbart invented almost everything presented here years ago." And he was right.
I hope a lot of you are watching The Mother of All Demos http://archive.org/details/XD300-23_68HighlightsAResearchCnt.... But it's worth reading and understanding the reasons why Doug was working on all of this.
Doug and his crew at SRI had the goal of "human augmentation". Everyone else at the forefront of the computer industry thought we'd have general AI by the 1970s. They instead believed that GAI wasn't within reach. They believed that the things we wanted to build and accomplish as a society weren't doable with the communication tools we had.
They had the idea that computers could be tools to help individuals work. Since computers were multi-million dollar calculating machines, the idea that people would have a computer at their desk and that they'd help us to communicate and manage information was beyond-out-there.
http://www.dougengelbart.org/pubs/augment-3906.html
But after leaving Engelbart's group at SRI, lots of his team joined PARC and built the modern GUI and networking.
For a history of the details, I highly recommend reading Markoff's What the Dormouse Said http://www.amazon.com/What-Dormouse-Said-Counterculture-Pers...
[Bonus: If you like Engelbart's MOAD, also watch Ivan Sutherland's Sketchpad Demo.]
I only tend to comment about once-a-year on HN, but it would be impossible not to say something about Doug and the impact he's had on our world.
Edit: Sutherland's Sketchpad http://www.youtube.com/watch?v=USyoT_Ha_bA