Three of the big legacy carriers have rows of economy seats in the front part of the economy cabin with extra legroom. United calls is Economy Plus. American calls it Main Cabin Extra. Delta calls it Economy Comfort.
There is an extra charge for the superior legroom unless you have elite frequent flyer status (and, even then, it depends on the airline and on your status).
I've also been successful asking nicely at the gate on United, even without status.
US Airways doesn't have it (yet). US Airways has ChoiceSeats, bit these don;t have better legroom, just more desirable locations within the cabin. US Airways is expected to mimic American's Main Cabin Extra as the US Airways brand goes away.