I really like this one from the NYTimes.com.  Converging Flight Paths visualizes the airline mergers over time from 1975 to today, but also shows the “Domestic share of total passengers flown each year.”

The deregulation of the airline industry in 1978 led to a wave of mergers that continues to this day. But even as the legacy carriers have been consolidating and growing, they have been losing market share to low-cost carriers. Two of them, SouthWest and AirTran, have just agreed to merge and carried the most domestic passengers in 2009 combined. But if international passengers were included, this ranking would be greatly rearranged.

By KARL RUSSELL/The New York Times.  Sources: American Transport Association (passenger data 1975-89); Bureau of Transportation Statistics (passenger data 1990-2009)

Link from Elliott Ng on Twitter.