It's hard to say: I could consider helping several ventures get off the ground but the truth is that every success is a team effort so I can't take credit there; I could talk about doing one of the first web server port or writing XML specifications when they weren't cool (eg. helping on RSS or the Creative Commons framework) but again, it's other people who steered those efforts successfully; I could talk about my 20 years of writing but I could not have written any of it without interviewing and being mentored by thousands, as well as the feedback of my readers.
I stand on the shoulders of giants and try to push things forward but there is no achievement I can lay claim to that is not the product of tireless work along with many others. Great teams are what I believe in and my ability to inspire or help them is my proudest accomplishment.
Describe the most impressive thing you've done.