I used to use ITA for fare searches but they were taken over by Google and now it's Google Flights.
You can search for flights from all of Europe using that tool. There's a feature that shows the lowest price flights in the menu someplace.
Difficult to cover "convenience" since you want to cover all of Europe and all of the USA.
Cheers
I remembered reading somewhere the ITA Matrix is somewhat different from Google Flights. For more info, see
What is the Difference Between Google Flights and the ITA Matrix?
From Travel.Stackexchange.com
"The ITA Matrix software was developed by MIT engineers.
ITA was then purchased by Google, and used to power Google Flights (as well as other
non-google flight search engines). Quoting from
the ITA website:
In the mid-nineties, a group of MIT computer scientists took aim at a complex computing challenge that had plagued the travel industry for three decades: how to quickly, consistently and accurately identify the best available airfares without relying on high-cost, low-efficiency mainframe computers. The results of their work was a revolutionary airfare pricing & shopping engine called QPX. The beta version of QPX was first introduced to the public in November 1999, and Orbitz became the first customer in 2001.
On April 12, 2011, Google acquired ITA Software, and shortly thereafter launched Google Flight Search, an incredibly fast and easy way to search and plan travel. So the Matrix isn't better nor worse than Google flights, since the former relies on the latter when looking for flights. It's just a matter of using one tool versus the other. Personally I stick with the Matrix because I've grown fond of the interface.
The main feature difference would be that you can purchase some plane tickets via Google Flights, whereas the Matrix itself does not allow you to do so. Moreover, the ITA Matrix seems to give you many more search options than Google Flights. Finally, Google Flights, being a Google product, will integrate with other Google products whereas the "pure" Matrix might not always...."
Richard