The trouble with all these sites, at least to me, is that they still make me do so much searching and research. And I still have to have a specific destination in mind. I'd say at any time I'm vaguely interested in hundreds of different trips: right now, I'd probably book a ticket to anywhere warm this weekend if I could find something for under $200, and I could justify flying out to SF if it cost less than $300, and I'm planning to go to Zagreb in the next two months so knowing when those prices are good would be useful. But I'm not going to sit at my computer typing in every beach destination to ITA's matrix, or setting up a Kayak alert for each one. We'd like Hitlist to be an ever-smarter travel agent who is monitoring all this stuff for me... and maybe in the future will also know about my airline alliance and stopover preferences too.
One use-case I often have, fwiw, is "what's the cheapest transatlantic flight right now?", which I have to find through trial-and-error. Once I know that, I can often find a cheap way to get to/from the relevant endpoints which doesn't show up on a regular fare search (train, bus, low-cost airline, etc.).
Of course a full end-to-end search that took into account all travel modalities and included all the low-cost carriers would be even better, but that seems like a more difficult information-aggregation problem.
Kayak Explore is pretty good for that. Select your city in the US, then check non-stop and Europe. It's on a map and I would prefer a list, but it does the job. Belfast and Copenhagen are the cheapest transatlantic in March, for instance.
