The following list is a dynamic one and should be visited frequently.
AS = Alai-his-salam, May Peace be upon him. Used for the Prophets (AS).
DB = Damat barkatuhum, May Allah Almighty prolong his blessings
RA = Radiallahu Anhu (or Anhum), may Allah Almighty be pleased with him (or them). This is used in case of the Companions (sahaba) of the Holy Prophet, may Peace and Blessings of Allah Almighty be upon him.
RA = Rehmatullah Alaihi, may Allah Almighty have Mercy upon him. This is used for departed pious Elders.
NM = Nawwarallahu Marakadahu, may Allah Almighy fill his tomb with light. This is used for departed pious Elders of even higher stature.
PBUH = Peace be upon him, this is used for the Holy Prophet, may Allah Almighty shower his Peace and Blessings upon him.
QS or QAS = Quddoos Allahu Siiruhu, May Allah Almighty sanctify his inner self. Used for the elders who are no more with us physically.