For me it was Google Wave. The presentation[1] got me very excited and I couldn't wait to start using it. I signed up for the beta and told everyone I work with to do so as well. It quickly became an invaluable tool for me, I couldn't imagine what I'd do without it. And then, a year after it was introduced, only 3 months after it was released to the general public, Google announced Wave was dead.
At least they open-sourced it and released a stand-alone version via the Apache Foundation. You can still use Wave, it's just called Apache Wave or Wave in a Box.
[1] http://www.youtube.com/watch?v=v_UyVmITiYQ