Some kind of Linux boot disk, with gparted, gpart and gnu ddrescue is invaluable. Some of the forensics tools for file carving can be good as a last resort. GetDataBack is very good as well. You might also want to keep copies of a few basic system files, like userinit.exe and ntldr, for when they get corrupted.
A couple of Windows install disks, for repairing installations of the different versions you support can be helpful. The System Rescue CD is also very good, it's got a load of useful Linux tools bundled into it.