Memtest and Memtest86
Memtest and Memtest86 are open source software utilities that scan and test computer memory chips (RAM) for any defects or errors. Memtest86 is a DOS-based program and includes a variation called Memtest86+. Both are actively updated to work with older 386 and 486 computers on up to the current dual and quad-core computers. It's compatible with Windows 95 through Windows 7 and allows you to define the amount of memory to be tested. Chris Brady originally developed Memtest86 and Memtest86+ was developed by Samuel Demeulemeester.
Also see: Memory definitions