Airfare Price History
At the website Fare Compare, you can search for airfares, and you can also do a price history analysis back for two years, by many different fare categories (7-day advance, 14-day, 21-day), and by different airlines.
For example, in the graph above (click to enlarge), it displays a one-year history of the lowest airfares from Detroit to Jacksonville, on NWA (blue line) and the lowest fare on any airline (red line). As you can see, the round trip fares over the last year ranged from $125 to $325, and were higher in the fall and spring than the summer and fall. Pretty cool.