I can't recommend High Intensity Interval Training (HIIT) highly enough. Its almost all I do for cardio, its so damn good. The catch is - it's hard.

Like most things worth doing.
I'll have to look up some studies and post them.
In one study, "The HIIT group lost over 3 times as much subcutanious fat as the ET group despite of only expending less than half as many Calories." See
http://www.exrx.net/FatLoss/HIITvsET.html
In another study, HIIT was found to be NINE times as effective as regular level-resistance cardio for fat burning.
I occasionally do sprints, more often I just do the "Hill" program on the older model lifecyle I bought on ebay. Two 18 minute sessions on level 9-11.