I was not impressed by my first look at Farecast (because their initial list of cities covered was so small) but after reading on TechCrunch that 'Farecast' expanded the number of cities you can search for airfares to 55, I decided to take a second look.
Since Newark (New Jersey), the nearest airport for me is one of them, it makes it easier to try.
I searched for a Newark (NJ) to Portland (Oregon) round trip fare (September 05-September 12), got a best fare (non stop) of $339.00, for more details here is the result. I like the 'Fare Prediction' and 'Fare History' functions.
Please note though that cities covered by 'Farecast' are all in the USA.
For foreign travel, I will still have to rely on Mobissimo or Cheap Flights.