Memtest86 is thorough, stand alone memory test for x86 architecture computers. BIOS based memory tests are a quick, cursory check and often miss many of the failures that are detected by Memtest86.
Memtest86 was developed by Chris Brady with a first release in 1994. However, the testing algorithms used have been under development since 1981 and have been previously implemented on Dec PDP-11, VAX-11/780 and Cray XMP architectures. Development and distribution of Memtest86 is now funded and supported by BradyTech Inc.
Memtest86 is released under the terms of the Gnu Public License (GPL).
Download: Memtest86 3.5 Windows, Linux and DOS