oh soooo too many to ever list, but for the USA, and a few more, lemme give it a stab at what I do know:

AA
DL
UAL
CO
NWA
US AIR
SOUTHWEST
HHILTON
STARWOOD
Baymont Guest Ovations
Jet Blue
Iceland Air
Singapore Kris Flyer
++++
One World
Star Alaince
British Air
Virgin
Quantas
+++
Click Rewards
Points.com
Milepoint

oh my ---forget it! this would go on FOREVER!!!!!!!!!!!!!!!!!!!

it's all 'mahhhketing,' man! The way of the age we live in!