[Jessica Boucher]

I had two events in my head when I first read this question.

1.) Alliances - Already stated by Steve & Wayne

2.) Rumble at the Rock. Although I never attended one, I feel that it single-handedly turned FIRST into a year-long thing as opposed to a Jan-April thing, as well as helped set the cornerstone for the FIRST community as we see it today.

Although I feel that there are other changes that have had great impact on FIRST, as for actual events...the shift to alliances and Rumble have had the most impact yet.