One downside to frequent-flier programs is being limited to one airline. The Global Points Exchange on, a loyalty program management site, lets members of participating programs trade miles. After signing up for a free account, users can browse an online trading board for listings or post their own. Users set the terms and pay the exchange about a penny for each mile swapped. So far, the exchange works with 10 major airlines, including Continental and American, as well as the InterContinental Hotels Group.