Needless to say, it comes from the USA.
In an article on SearchEngineWatch, Brian Smith highlights the latest developments in air fare search and two sites in particular...
Farecast.com tells travellers when to buy tickets to get the best deals. The site tracks airfares and using "data-mining algorithms to search for patterns" it predicts whether the fare for a particular route is about to go up, or down. At the moment the 'beta' site covers routes from 75 airports in the USA, but no doubt it'll migrate over here one day, or somebody else will do it.
Farecompare.com does work in the UK. It simply lays out the lowest fare on a route (eg. London - Los Angeles) over the coming months so you can see which month is most economical to travel in.
Like all good ideas, both are simple and cause you to wonder why somebody didn't think of them before.