Which world most important algorithms have contributed most to humankind in past decades? 

I thought this is a good general knowledge for a developer to know about.

Update:
If possible, please keep the answer to a specific programming algorithm.
I would like to get a list of the most important ones, only one algorithm per answer.
Please consider to state why the algorithm is significant and important...

Public/private key encryption is pretty darn important. Internet commerce would be nowhere as ubiquitous without it.