Completely different field in physics, but I can't help be excited!
A century after a prediction by Einstein, observations for 13.5 years, and development for a decade longer, finally gravitational waves were seen on September 14, 2015 @9:51AM UTC (4:51 pm EST).
Of course, there is more to this than just the confirmation of a prediction, this could usher in an era of gravitational wave astronomy. Though seems like the events have to be pretty big.I can't think of a more significant discovery in physics in my lifetime.
It is also a testimony to the power of "big science" and collaborations. 1004 co-authors and take a look: