http://www.seatguru.com/ has some comparison charts, as well as seat maps by airline and aircraft type.

Most airlines in the US have 31" seat pitch in economy. United's Economy Plus is usually around 35". Jet Blue's A320 fleet has 34" (only 32" in the E190 fleet) unless you buy up to the Even More Legroom seats with 38". Southwest is mostly 32" to 33", but there is one seat with no seat in front of it (the row in front is an exit row without a window seat), so if you get an early enough boarding pass (either by paying for it or flying enough to get the needed status), you may be able to get that seat.

One other note on United is that their regional jets include CRJ-700 planes with what they call "explus" seating. Basically set up where nearly half of the economy section is Economy Plus. It is often the case that the regular economy will fill up, so that non-elites will not be able to select a seat at booking; at check-in, they will often be given Economy Plus seats even if they do not pay the buy-up fee.