Kellogg's Family Rewards --- use MYFAVCHARACTER50 for 50 Rewards points.
Codes that are still valid:
JOINKELLOGG2SAVE - 50 points (works even with existing accounts, though meant as a sign up code, expiration unknown)
HOWITWORKS20PNTS -- expiration unknown (20 points)
KFRFIBERPLUSPOLL - expires 12/31/12 (10 points)
If any are expired, let me know in a comment below so I can strike them off the list. Thanks!