iOS: While there are lots of things you can do to find the cheapest airfare, why not put technology to work? Hitlist calculates when it's least expensive for you to grab a flight to your pre-picked destinations, saving you the hassle of periodically checking travel search engines.
Hitlist hooks up with your Facebook and gathers its data from there. It will show you the different cities that your Facebook friends live in (along with a cool photo of that place from Flickr) and ask you to specify whether you've been there, and would you like to go there or skip it. Add it to your hitlist and you're done -- the app will not track airfare flights and find the best possible time for you to go.
You can also search for destinations that your Facebook friends aren't in and add them to your hitlist. There's a cool world map to show you all the places you plan to go to or have visited already. Hitlist includes a social element for you to compare with your friends where you have gone or not.
The app is pretty customisable too. You can pick the best airport for you, specify whether you want to travel only on weekends or whenever, make your hitlist private or public, and set a budget for your tickets.