memtest86
Memtest86 is an open source diagnostics utility for testing a computer's random access memory (RAM) on an x86 machine.
First developed in 1994, the program creates test patterns and reads the written data to check for errors. Depending on the memory installed on the computer, the memory tests can run for a few minutes or a few hours.
This Defcon Computers video demonstrates how to test RAM with Memtest86.