In lieu of tips in Cambodia to those who are not normally tipped, one can make a contribution to places like the Ponheary Ly Foundation or make some microloans via Kiva and their Cambodian partners (see more in FlyerTalk Cares! Forum), or contribute to the amazing KANTHA BOTHA hospitals founded by Dr. Beat "Betocello" Richner (see
here).
Any of these will touch a number of lives and have a lasting effect; it certainly is what I have done, and continue to, do years after being in Cambodia.