Luckily I attended Microsoft Mix'06 conference in Las Vegas in March, 2006. As the conference included various topics covering web design, web development, smart client application, next generation of OS and browser, so the conference was named Mix (mixture).
I was quite impressed by the demos of Windows Presentation Foundation, Windows Presentation Foundation Everywhere, and Atlas. I think other sessions must be also interesting, such as "building your own search engine", but I couldn't attend those meetings at that time.
But now, I can watch all those sessions online :)