I'd recommend www.skyscanner.net, at least for the European users.
Sure, they lack simplistic interfaces of Hipmunk or Kayak but when it comes to actually finding the cheapest tickets and best connections on the old contintent low fare airlines market they're, IMHO, the most reliable.
I'm a big fan of skyscanner. Being able to ask questions like 'what are the cheapest flight from my local airport over this weekend' make for fun trip planning.
