Being only 27 I haven't encountered much at all.
Peace on the island of Ireland (1999, GFA).
9/11 and war and unrest in the Middle East and Asia.
Introduction of the Euro and abolition of the Punt.
Enlargement of the EU.
The state visit of the monarch of England (IMO a huge and symbolic event).
and that's all of significance really.
How old are you and what have you witnessed in your life?