In 1968, Doug Engelbart led the most famous computer demo of all time. With his team of seventeen researchers working behind the scenes, Engelbart showed off an interactive online system which had been in development since the early 60's. The demo was the coming-out party for the computer mouse (which Engelbart invented, with a colleague), hypertext, screen-sharing, computer-based videoconferencing, and more.
You can view the full demo on Google Video, or view an annotated version broken into sections. You may also be interested in Wikipedia's page on Doug Engelbart, which includes an image of the early mouse design.
Nearly 40 years later, Engelbart is still at work on advanced hypertext systems. His online HyperScope system is a first step towards his vision for computer systems of the future. One notable trait of Engelbart's work is the notion that systems should be expert-oriented -- meaning that improved productivity for system experts is the priority, rather than ease of use for beginners. Take one look at his demo, and you'll see the power of an expert user working with a powerful computer system.