There are two instances that immediately come to mind. The first: Boise State beating Oklahoma in a nationally televised bowl game. The second: Appalachian State beating Michigan at the Big House.
These were two victories which—despite my lack of knowledge or passion for college football—actually sparked my interest.
The Boise State game was extremely exciting to watch. The Buster Broncos were able to make an otherwise uneventful bowl season into an awesome night of entertainment. The team played with reckless abandon, and a joy that is seldom seen at that level of sports.
With Boise State's victory notwithstanding, the actual underdog victory that I imagine will be talked about for decades is that of Appalachian State over Michigan. This victory had additional value to me, as I was golfing with a couple of my friends who have been lifelong blue fans—suffice it to say, the post round meal was not full of cheerful conversation.
However, what sticks out the most about this victory is that the feat had never previously been accomplished. That is, no "Football Championship Subdivision" (FCS) team had ever beaten a ranked "Football Bowl Subdivision" (FBS) team. In fact, because of this once in a lifetime victory, the Associated Press altered their policy, allowing for FCS teams to be ranked along with FBS teams in their weekly polling.
But the story does not stop there. On opening day of this season, Appalachian State not only did something that has never been done before, but the team also went on to win its third consecutive FCS championship.
The Mountaineers football team has been nothing short of amazing. Combined with Boise State’s victory over Oklahoma, the Broncos and Mountaineers have given hope to all of the mid-major and "cupcakes schools" for success against "the big boys."