For me, 9/11 and the Y2K bug are among the various events that define the 2000's.
But we shouldn't forget Hurricane Katrina, The Boxing day Tsunami, The 7/7 London bombing, The Mumbai attacks, Flight 1549 ditching in the Hudson River, and President Obama's inauguration among other noteworthy events. I think some people will consider the 2000's as the best decade so far for their own reasons and others will not.
On a lighter note, I still have the Y2K Bug on computer:

