Hitlist (hitlistapp.com) is also designed specifically with flexibility in mind - we don't have every single route on every day, but we let you set an alert with whatever criteria you have in mind - from 'SFO to London on these days +/- 3 days' to 'a beach in southern Europe for 7-10 days between January 8th and 31st' - and can ping you when fares that match come up.