A friend called today to ask how much a ticket would cost to fly from Boston to Chicago. I told her that I had no idea, and she was shocked. She evidently wasn't familiar with the Marry Me, Fly For Free concept. Actually, this happens frequently. People assume that I must not all the various fares between myriad cities, including those on airlines that I don't work for and cities that I don't fly to.
On the occasion that I buy a full fare ticket for myself, I check the usual travel sites, just like everyone else. The individual airline websites are getting better about listing lowest priced fares, and some guarantee that the price you get from them is the lowest.
Well known major travel sites that I check include: Expedia, Orbitz, Hotwire, Cheap Tickets, and Travelocity. I have found fares on these sites to be fairly consistent with one another.
Other sites that I check, that are perhaps a little bit lesser known are: Side Step (compares the fares of several different carriers), Which Budget (a listing of the budget carriers that fly between various cities), and Fare Compare.
Yes, I know there are lots of other sites, but these are the ones that I use because they've always worked well for me.