MemTest is the best ram-testing utility for the Macintosh platform with the ability to find intermittent memory problems that are the frequent cause of unexplained crashes or freezes in Macs. Memtest can identify random memory problems that commercially available utilities miss completely including defective motherboard CPU cache memory.
MemTest is distributed as a universal (fat) binary containing four separate executables targeting each of the currently available Mac platforms. The appropriate machine-specific executable is loaded by OS X at launch time.
Memtest executes in 64-bit mode on PPC G5, Intel Xeon-based Mac Pros, and the latest Macbook Pro Core 2 Dual systems
running MacOS 10.4 or later, allowing virtually any amount of installed ram to be tested. On all other systems, memtest executes in 32-bit mode with a maximum testable limit of 2 GB.