United and Northwest have reintroduced the “Fly three, get one free” promotions that caused such a stir earlier this year.

United actually led the charge, offering a free domestic economy roundtrip ticket for every three paid qualifying roundtrip flights flown through Jan. 31, 2005, for travel until June 10.

Qualifying tickets may be booked in any paid booking class in United First, Business or Economy class with a minimum roundtrip purchase of $250 or more. Economy-class travel (in anything other than Y or B classes) cannot include a Saturday-night stay.

A maximum of four free tickets may be earned.

Not to be left behind, Northwest Airlines soon thereafter launched its latest “1-2-3 Free” promotion, with essentially the same rules — sans the $250 minimum and Saturday-night stay requirements.

Northwest sweetened the deal, though, by offering a free international ticket (up to a maximum of two) for every six qualifying roundtrips (members must opt for either domestic or international free travel). Free domestic or trans-Pacific travel must take place between Oct. 15 and June 10, 2005. Free trans-Atlantic travel must be completed between Oct. 15 and May 23, 2005.

Registration for both offers is required by Nov. 30.