If you're a frequent traveler, you may already be aware of this little tool. I NEVER book a flight without checking with them first. It's http://www.itasoftware.com/ and is a reference for flight costs across the board. It is not a booking tool. The concept is similar to the programs used by travel agents.
1. Choose "Search Fares"
2. Sign in as a guest
3. Plug in your info - as detailed or open as you choose
Voila...you'll see the costs, various airlines, flight numbers, stopovers, dates, times, etc for your cities. You then take that information and book through your preferred outlet or the airline directly. This way, you have the flight numbers and connections that create the lowest priced fare available.
The screenshot below is a search I did using the "month long" search to find the cheapest dates to fly in April. Flying on a Monday in April (vs. a Tuesday) saves close to $1k a pop per person. Crazy.p.s. Not that anyone is taking JetBlue to Addis, but in case you want to use ITA for domestic travel - JetBlue is not in their system. I think they may be eventually.