Jef was busy developing Archy, a new kind of user interface based on the design ideas and principles described in his book, The Humane Interface. Archy is quite different from what most of us are familiar with, and i didn't agree with all the design decisions he made, but the design certainly contains a great deal of valuable wisdom. I had the good fortune to see him at Foo Camp last year and enjoyed debating some of those design decisions with him, and am sorry that we won't get to do that again.
Perhaps a lesser known part of Jef's life were his interests in light radio-controlled airplanes and in music. When i met him for the first time at a Foresight Institute event, he talked to me about conducting. He said that conducting was one of the greatest challenges one could experience.
He's written some insightful articles about a variety of topics: not just interface design but also education and politics. Go on over to his website and check them out — they're listed on the right under "Published Works".