If you haven't seen it yet, John's WikiWikiWiki page has become a memorial, with posts from those who were fortunate enough to have worked with him.

Being a computer science student, I knew of him, and I had seen him a few times at OOPSLA. He was conference chair of OOPSLA 2004, and I held a junior committee position that year. I was nervous about working with him; after all, if we can say that CS has rock stars, one of the members of the Gang of Four has to qualify. I was surprised when I found him to be immensely approachable. He gave me enough leeway to run my portion of the conference as I saw fit, and was very careful to ensure that I knew that I had his support in all of my efforts.