Was one of the first I was going to mention. Being on the board of the NY Tech Meetup, and in the room for the demo, it was electric. Set the bar for every similar demo that followed.
Personally I saw Dropbox launch there and was blown away by it. The utter simplicity of the interface and just drag and dropping into a web page, for the time it was ground breaking. Iirc they were preceded by a guy demoing an alarm clock webpage and just the contrast was striking...
