Flyspy’s current chart of flights between Minneapolis (MSP) and Chicago’s Midway Airport (MDW) illustrates the Southwest Effect:

Things get interesting on March 8th.
Flyspy continues to impress the crap out of me. Especially their Need Miles page where I can see how much it costs to go to a wide variety of destinations from MSP on a single page. There are times when I just want to get out of town, on a beach, to a different city, etc., and I don’t really care where as long as it’s affordable. Flyspy is the only site I know that makes it easy for me to figure what my best options are.
Kayak.com has their “Fare buzz” page, where you can see others’ searches and filter by months/continents.
I play with it a lot and have booked some cheap fares by using it.