First one that comes to mind:
1993 World Series, game 6: Philadelphia Phillies at Toronto Blue Jays
Bottom of the ninth, two men on base, Toronto's Joe Carter steps up to face Phillies reliever Mitch Williams. The Blue Jays are losing the game, but they're up 3-2 in the series. Next pitch: Carter blasts it over the left-field wall to win the Jays' second consecutive World Series.
Others worth mention:
- 1993 Stanley Cup Playoffs: Toronto Maple Leafs' rookie Nikolai Borschevsky scores the OT winner in game 7 to knock the heavily-favoured Red Wings out of the playoffs
- 1996 Super Bowl: Brett Favre leads to the Packers to their first championship in almost thirty years.
- 2003 NFL Week 16: The greatest, most memorable individual performance I've ever seen. The day after his father passes away, Brett Favre chooses to play in a Monday Night Football game versus the Raiders. He throws for 399 yards and 4 touchdowns in the first half, and the Packers win 41-7.
- 2003 NFL Week 17: After it appears the Packers are eliminated from the postseason, some schmuck Cardinals wideout named Nate Poole makes a miraculous grab at the end of regulation to defeat the Vikings, and give Green Bay the division title.
I'll think of some more later.