JIM DOWSON  A complete waste of time- don't bother.

All this does is redirect your default web browser to a site, with the partial data filled out.

It's not eve worth 'free'.  
(Version 1.8)

ANONYMOUS  Tried it and it worked. But! Even though it worked on European Domestic Flights with acceptable results, it didn't pick the most cheapest flights. That's because within the Database used are significant European Airlines (e.g. RyanAir, hlx, AirBerlin, Air Lingus, dba) missing. Anyway if you're not European and don't know about the alternatives it pretty much suites your needs.  
(Version 1.0)

AVFOLK  Superbly done. Easy to input cities by airport code. Database returned some impressive bargains for travel only a few days away. Excellent, clean graphics and small size make 'Air Compare' a MUST HAVE widget for those who travel.   
(Version 1.0)

KARL BROWN  works pretty well saves a lot of time   
(Version 1.0)