The following cards can net savings (usually about 10%) on museums, accommodations and some transport (including Amtrak):
American Association of Retired Persons (www.aarp.org) For US travelers age 50 and older.
American Automobile Association (www.aaa.com) For members of AAA or reciprocal clubs in Europe and Australia.
International Student Identity Card (www.isic.org) For students of any age and non-students under 26.
Student Advantage Card (www.studentadvantage.com) For US and foreign students aged 16 and older.