I use the websites of particular airlines or
www.opodo.co.uk for checking prices anywhere in the world.
http://dps1.travelocity.com/lognlogi...tr_module=FARE shows virtually all published fares for any given city pair (unfortunately lo-costs aren't included AFAIK), so you can see whether fiddling with travel dates has any potential to make a difference on the cost of your tix.