FarePlay is a free flight search site that finds cheaper trips by splitting tickets: instead of one expensive round-trip bundle, it compares separate one-way tickets (on any airlines, and through other cities) and shows the cheapest combination, with the total price for all travelers and a link to book each exact flight.
FarePlay splits tickets to save money and bypass expensive round-trip bundles.
It searches every leg of a trip separately (out, home, and the flights between destinations on multi-city trips), then compares every combination of days and airlines.
When one round-trip ticket is cheaper, FarePlay shows that too, so you always see the lowest total price.
It is free to use. FarePlay does not sell tickets: it links to the airline or travel agency that sells each flight.