I have been using Memtest86+ for all the ESX servers I deployed. It has been all good, but it requires booting from Memtest86+. During the memory check, ESX server isn’t running. Ran across a tool posted in xtravirt.com, which is actually an ESX built-in utility to do memory check. The command is ramcheck. The good thing about this is it is a built-in utility, and it runs in the background of ESX, which means no down time will be involved. Just run
service ramcheck start
It runs in a background and writes out a log file to /var/log/vmware/ramcheck.log and ramcheck-err.log. Since it not only checks the memory in Service Console, but also the memory in VMkernel, it has to run as a world in VMkernel space. It consumes only a few percent of CPU resource in the background, but the trade of is the time will take a couple weeks to complete. Run esxtop will show it as ramcheck.<id>. It will stop automatically once it finished all the memory check.
It might not be a complete replace of Memtest86+, but it does offer an alternative, non intrusive maintenance memory check during operation of an ESX server.