An INAPAM card is an ID card issued by the Instituto Nacional de Las Personas Adultos Mayores identifying the bearer as qualified for certain senior citizen's discounts ranging from 50% fare reductions on interstate luxury buses to museums, galleries, national parks, etc.  The savings on long-distance bus trips can be substantial.  Some bus lines limit the number of "INAPAM seats" available on a bus.

The INAPAM card will also get you discounts on prescription drugs at certain pharmacies such as most big box stores. In addition, some cities offer a 50% discount on property taxes and water bills.
 
The card may be obtained from your local DIF office.  The card is free.  You must be at least 60 years old.  You will need your birth certificate, passport, visa card (not a Visitante), proof of address, and some pictures.  The DIF office will tell you their exact requirements. You will need a CURP.

Their website is here.  English versions are available for most pages.