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https://matrix.itasoftware.com/ can search multiple airports (both arrival and departure). You can either search +- 2 days or an entire month.
You can also specify (or exclude) specific airlines, alliances, layover cities, duration limits, etc.
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Upvote:7
Kayak will do this for you (at least for me, sitting in the USβits capabilities may be different elsewhere.) For example, here are the results for a 6-night round trip from the NYC area to the LA area.
You do have to visit an external website (the airline or another third-party reseller) to actually book the ticket. Unlike the ITA Matrix, though, Kayak provides the necessary links.