Here's a challenge: what combination of FF programs will enable you to fly the widest range of airlines while still avoiding dispersing your miles too widely? The goal is to get as many miles as possible on as few programs as possible, while still being able to choose which airline to fly based on price.

The combination that I use:
Northwest
US Air
Continental
Delta

I also am members of United and American's FF programs, but have never actually found it necessary to fly them. The *vast* majority of my business goes to Northwest and US Air.

Continental is primarily useful because of its partner agreements with Frontier and America West. Northwest is great for a lot of reasons, but among others for its partnership with Sprint and Sprint PCS. US Air is a great airline that often has creative bonuses.

My set of six airlines would enable you to fly *almost* any other airline and still get FF miles. Anyone care to propose an alternative set of airlines? More/fewer?