It's called HipMunk and for the time being* it has changed our lives for the better.
Why we love it:
* It shows the flights rated by "agony" which is a combination of price, number of stops, length of layover, total flight length etc!
* It show the flights horizontally over a time frame, so that it is super easy to compare the different options visually!!!!
* It allows you to open multiple tabs in the same window, and remembers your previous inputs so that all you have to do is change the dates by a day or two!
We used hipmunk to book our flights and it was super easy- once we had mulled over our 18 tabs of dates, we just clicked on the chosen flights and were taken to the booking site (in our case Orbitz).
Hope you find it useful!
*Why for the time being? Because most search engines start out really good finding great deals. As they grow and make partnerships with other travel organizations they find flights that are lucrative for them rather than the best deal for us. Kayak being the most recent example.