AIDA32 is the ultimate information utility! It doesn't have to be installed on the system, you can burn the AIDA software to a CD, pop it into a customers system, run it, and know every piece of hardware and software in the system. It will even find product keys for most MS software and some other applications.
Powermax will give you all the diagnostic info on Maxtor hard drives, along with generating RMA codes for bad drives.
DataLifeguard Tools will diagnose, partition, format, and copy your old drive to your new WD drive.
Seagate's Seatools are 98% accurate (acording to them

) on Seagate drives.
DocMemory is a great memory testing utility that loads from a floppy, so there's no OS interference
Anybody else have some handy utilities to add?