i find skyscanner to be far superior. kayak's is a great overview but i didn't find it actually took me straight to when those cheap flights were - you still had to guess the cheap dates. when you know where you want to go and just want to find the cheapest time to fly: it shows you the flight costs over a month, and you don't have to hope that the fare is still available.

example of nyc to chicago over month of june: http://www.skyscanner.com/flights/nyca/chia/june-2010/june-2...

see smaher's comment for an even better idea!