Four of the six biggest levels of internet usage occurred at the 2010 World Cup

The highest recorded internet usage of 20.7m visitors to news sites per minute was on 24th June 2010, when Denmark played Japan, and Cameroon played the Netherlands, while the longest running tennis game was taking place at Wimbledon.

The top 6 events are shown below

Click to enlarge

Source: Akamai Net Usage Index, retrieved 14th July 2010
From their notes:
"The highest peaks since inception of the Net Usage Index for News (August 18, 2005), based on total visitors per minutes. The following data ranks the top peaks, and the corresponding events that were taking place in the world at the time."